The Hong Kong flu, also known as the 1968 flu pandemic, was an influenza pandemic that occurred between 1968 and 1970 and which killed between one and four million people globally. It is among the deadliest pandemics in history, and was caused by an H3N2 strain of the influenza A virus. The virus was descended from H2N2 (which caused the Asian flu pandemic in 1957–1958) through antigenic shift, a genetic process in which genes from multiple subtypes are reassorted to form a new virus.

History

Origin and outbreak in Hong Kong and China

The first recorded instance of the outbreak appeared on 13 July 1968 in British Hong Kong. It has been speculated that the outbreak began in mainland China before it spread to Hong Kong: On 11 July, before the outbreak in the colony was first noted, the Hong Kong newspaper Ming Pao reported an outbreak of respiratory illness in Guangdong Province, and the next day, The Times issued a similar report of an epidemic in southeastern China. Later reporting suggested that the flu had spread from the central provinces of Sichuan, Gansu, Shaanxi, and Shanxi, which had experienced epidemics in the spring. The reported data were very limited due to the Cultural Revolution, but retrospective analysis of flu activity between 1968 and 1992 shows that flu infection was the most serious in 1968, implying that most areas in China were affected at the time. However, they were not at the time considered to be an entirely new subtype of influenza A, only a variant of older strains. Nevertheless, the World Health Organization warned of potential worldwide spread of the virus on 16 August. and Malaysia, and, before the end of the month, an epidemic was underway in the Republic of Vietnam.

In September 1968, the flu reached India, northern Australia, Thailand, and Europe. The same month, the virus entered the United States and was carried by troops returning from the Vietnam War, but it did not become widespread in the country until December 1968.

In the USSR, the first cases of the flu began to appear in mid-December.

It reached South America by 1969.

The development of the pandemic at first resembled that of the 1957 pandemic, which had spread unencumbered throughout the spring and summer and had become truly worldwide by October, by which point nearly all countries were experiencing their first or even second wave. However, the two experiences eventually diverged within a couple of months after their initial outbreaks. In 1968, many countries (e.g., the United Kingdom, Japan) did not immediately see outbreaks despite repeated introductions of the virus throughout August and September. Additionally, after September, there was little evidence of continued spread in new areas, despite similar importations of the virus into those areas. Epidemics did eventually develop during the winter months, but these were often mild (especially when compared to the experience in the United States). The name "Hong Kong flu" was not used within the colony, where the press dubbed it the "killer flu" after the first several deaths. China certainly did not escape associations with the new virus, however, as the name "Mao flu" suggests. It was speculated even at the time that the virus had originated from "Red China". In addition to these names, the virus was also often referred to as "Asian flu" or "Asiatic flu", as it was not yet considered an entirely different subtype from the previously circulating influenza A.

Worldwide deaths from the virus peaked in December 1968 and January 1969, when public health warnings and virus descriptions had been widely issued in the scientific and medical journals. Isolated countries like Albania reported the first cases of the flu in December 1969, reaching a peak in infections in the first months of the year 1970. In Berlin, the excessive number of deaths led to corpses being stored in subway tunnels, and in West Germany, garbage collectors had to bury the dead because of a lack of undertakers. In total, East and West Germany registered 60,000 estimated deaths. In some areas of France, half of the workforce was bedridden, and manufacturing suffered large disruptions because of absenteeism. The UK postal and rail services were also severely disrupted.

Asia

India

On 8 September 1968, the ship S.S. Rajula docked at Madras (present-day Chennai) from Singapore, where influenza had been epidemic the month before. On board were 16 people who were suspected to have influenza infections, a number of which were confirmed to be caused by the pandemic virus. On 9 September, only eight cases of typical influenza were reported in city hospitals, but by 17 September the number had reached 7661, after which incidence of the disease declined to just 68 cases on 31 October. An outbreak of influenza soon erupted among the participants, afflicting at least a third of them. The convention was the apparent origin of a broader outbreak within the capital city, which thereafter spread rapidly throughout Iran. It was not until January 1969 that the outbreak began to develop the typical features of an epidemic among the general population, although it remained moderate in its extent and spread less rapidly than expected for an outbreak of a novel influenza virus.

During this same period, an epidemic of influenza B affected the whole country, including rural parts. For this reason, it is difficult to ascertain the extent to which the pandemic virus and the seasonal B virus each contributed to the general wave of influenza-like illness that occurred between January and February 1969. Like that in many countries of Europe, the second wave in Japan was more severe than the first. While the 1968–1969 wave saw excess pneumonia and influenza and all-cause mortality rates of 5.4 and 27.0 per 100,000, respectively, the 1969–1970 wave saw rates of 11.3 and 49.1 per 100,000, respectively. In sum, approximately two thirds of the total pandemic mortality occurred over the winter of 1969–1970. In Sydney, the disease broke out at the beginning of July and continued into August. Another epidemic, more widespread and severe than the first, afflicted the Northern Territory in 1969. As the disease spread throughout the Pacific region and around the world, the Territory's Department of Health took the opportunity to observe the situation in other countries and ensure treatment services were available to respond to outbreaks in rural areas.

In August, the South Pacific Games were held in Port Moresby from the 13th to the 23th and later the Mount Hagen Show was held in the Western Highlands on the 30th and 31st. These two events, the latter seeing an attendance of nearly 100,000 people, were likely significant in spreading the disease and reigniting the outbreak. On 29 August, pandemic influenza was reported as "raging" in Lae (on New Guinea itself), in Rabaul (on the island of New Britain), and on Bougainville Island. However, the disease was described as mild, with its main symptom being a "very sore" throat. No deaths had thus far been reported.

The epidemic raged practically unabated throughout the Highlands, where the disease "spread like wildfire".

By 13 October, the death toll had risen to "at least" 235, though unconfirmed reports out of Goroka indicated that the toll had already reached 500 in the Southern Highlands. By the next day, the toll had climbed to 403, 207 in the Southern Highlands, and now health authorities could see no end to the epidemic in the near future. It had reached the Eastern Highlands and Morobe Districts, and it was now expected to reach not only Madang but Sepik as well later in the month. Patrols were on the move into all villages in the Highlands, and extra supplies and personnel were being directed to aid stations in areas of most need. By the next morning, the number had reached 462, It was now clear that not many areas would not remain unaffected. In response to this unexpected severity, it was announced that the Health Department had intensified its efforts to combat the epidemic. These new measures involved alerting and resupplying all aid posts via personal visit, doubling staff at aid posts where the disease was occurring, and continually visiting aid posts over the next several weeks.

Health authorities, with the help of the Army and its helicopters, spent the weekend distributing and administering vaccine. No new deaths were reported, though Roy Scragg, Director of Public Health, acknowledged reports of further but as of yet unconfirmed deaths. More deaths indeed were soon to be officially reported.

On the morning of 21 October, widespread reports indicated that the death toll from the epidemic had surpassed 1,000, though no confirmation came from the Administration. In the afternoon, it came to be known that the Department of Information and Extension Services would be releasing a statement on flu deaths, but upon being contacted, the Department claimed the information was not available. This was, apparently, because the Department of External Territories first had to approve the release of the information. Territory Administrator David Hay had no comment when asked about this confusion. At last, sometime after 6:30 pm, the updated death toll of 1,455 was released to the public.

This sudden increase in deaths over the week before was not necessarily due to an increase in the severity of the epidemic but rather because reports had finally come in from patrols moving through remote areas, and for the same reason, the death toll was expected to rise again. Still, it was evident that the outbreak was one of the worst to affect the Territory in recent years, now affecting all 18 districts, and it was worsening still in the Southern Highlands, where 600 had died in just three weeks. In light of these developments, the Administration called for outside aid for the first time. A Royal Australian Air Force aircraft was now on the way to deliver an army hospital to assist medical authorities in their efforts to combat the epidemic.

On 22 October, it was reported that the epidemic would likely cost more than $250,000 in vaccine supplies and drugs alone, while the cost of helicopters and other means to contact people in the immunization campaign was not yet known. Hay further announced the establishment of a five-man Epidemic Relief Committee to determine the relief measures required by affected families and to direct aid where needed.

In Lae, Dr. C. Matthews, Medical Superintendent of Angau Memorial Hospital, announced that no special precautions would be taken during the upcoming Morobe Show, to be held from Friday the 24th to Sunday the 26th. He explained that all possible precautions had already been taken to immunize people in the villages and that to set up a vaccination program at the show "would be merely doubling up". The alternative, to cancel the show altogether, was considered earlier in the week but was ultimately decided to be unnecessary. The next day, it was reported that tests were in process at the Commonwealth Serum Laboratories to determine if a new virus could be responsible for the severity of the epidemic in the Highlands, in particular the disproportionate number of deaths among young adults in the area, similar to what had originally happened in Hong Kong the year before, when the pandemic virus was first detected.

After the report of the first case of influenza in the Tari area of the Southern Highlands on 24 October, Administration officials and staff worked to seal off mountain passes between the area and infected parts on the other side.

Lokoloko and the rest of the Epidemic Relief Committee returned to Port Moresby on 29 October following their inspection tour of the Highlands. They reported that some areas of the Highlands were experiencing a severe food shortage, in particular around Pangia, at the eastern end of the Southern Highlands. While adults there typically consumed nearly 11 lbs of kaukau (sweet potato) per day, some were now subsisting on as little as 6 oz per day, and a few had not eaten anything for several days. In response the committee arranged for 10 tons of rice and two tons of fish to be sent to the area at once, and shipments of the same amounts would continue periodically until new gardens began producing food.

On 30 October, the Director of the Commonwealth Serum Laboratories, William R. Lane, announced that tests performed on virus samples isolated from the epidemic showed that the strain responsible was in fact the Hong Kong flu and not a novel virus. Scragg expressed his relief at this news, as that was the type for which medical officers had been treating the people of the Territory.

The next day, Scragg announced that the epidemic death toll had reached 1,980 but that the worst now appeared to be over. Of the new deaths reported, 42 had occurred since 20 October, across Morobe, Madang, and the Southern Highlands. He reported that the situation in the Southern Highlands was now under control. On 3 November, it was reported that the Army units that had been helping in the fight against the epidemic were beginning to move out, though some would remain and be moved around mostly to assist in vaccination rather than treating the sick.

On 7 November, Scragg reported a further 33 deaths due to the epidemic since 20 October, bringing the death toll to 2,013. In total, only 75 deaths had been reported since that date, and Scragg stated that the death rate due to pneumonia and influenza appeared to have returned to normal levels in the affected areas. The disease was still occurring in some small pockets, but few deaths were being reported. Dr. A. L. Malcolm, the Regional Medical Director at Lae, reported that the epidemic appeared to have paused and that officers were now waiting to see how it would continue.

No further deaths were reported over the weekend, it was announced on 10 November. Now, senior Health Department and Army officials were moving into the field to visit affected areas and to decide when the Army could pull out and the Department could effectively take control of the epidemic response. On 11 November, the Army continued its pullout from the Highlands, with most Army medics set to move out on the 15th. However, some forces would remain in West Sepik and other areas of Sepik for the time being. In response to these developments, Scragg stated that this withdrawal of the Army was a definite indication that the epidemic was on the decline and that the Administration was now able to handle it on its own.

On 13 November, Lokoloko addressed the House of Assembly regarding the influenza epidemic. He described the outbreak in the Southern Highlands as "a major disaster" and explained the necessity of the assistance from the armed forces. He further recounted how the Health Department had prepared for the epidemic and why vaccination had been advised against at first but then later recommended. He explained health officers' view on why the Southern Highlands had suffered so severely in comparison with other areas. In short, this was due to the low temperatures of the Highland plateaus, malnutrition, the people not seeking medical attention when it could have been provided, and widespread sickness that incapacitated whole families at the same time. He reported that about 2,000 had died in September and October out of the one million people living in the Highlands, though it was not possible to know for sure how many had died from the epidemic. He described the experience of one area where the number of deaths reported was twice the number that had actually occurred in September and October, and some deaths were in fact from up to two years before. Nonetheless, he asserted that despite these issues in reporting, the number of deaths due to pneumonia and influenza was undoubtedly at least twice the number to occur normally over the same period.

On 18 December, Reuben Taureka, Highlands Regional Medical Officer, announced in Goroka that the influenza epidemic in the Highlands had concluded. At a meeting of the Southern Highlands District Advisory Council on 6 January 1970, Fr. Berard Tomasetti of Pureni moved a vote of thanks to those who had helped in the fight against the epidemic in the Highlands.

Europe

France

Pandemic influenza was first noted in France in the latter half of January 1969, with sporadic cases appearing in the district of Paris. It thereafter spread throughout the rest of the country, though incidence of the disease remained low and moderate in extent. In many areas only sporadic cases were noted, although there were some localized outbreaks reported in towns and villages. The outbreak reached its peak in March, began to decline in April, and subsided in May.

The disease was generally mild or moderate in severity. Although some months saw greater mortality than the corresponding periods of 1967 and 1968, the total number of reported deaths due to influenza during the first 4 months of 1969 was only 3186, as compared with 3472 and 7267 in 1967 and 1968, respectively. Excess pneumonia and influenza and all-cause mortality rates in 1968–1969 were 6.2 and 21.8 per 100,000, respectively, as opposed to 16.6 and 58.9 per 100,000, respectively, in 1967–1968. Nonetheless, as in other European countries, the second wave resulted in greater mortality in France as compared with the first, with excess pneumonia and influenza and all-cause mortality rates of 35.3 and 71.5 per 100,000, respectively.

Italy

Pandemic influenza was not detected in Italy until the last week of February 1969, from a case in Genova. The country saw only sporadic cases of respiratory illness in the winter of 1968–1969, and there was ultimately no epidemic that first pandemic season.

The disease began to spread with force in late November 1969, around the time that the Apollo 12 crew returned to Earth; for this reason, some Italians humorously designated the outbreak "the moon flu". By 9 December, a "fairly large epidemic" had spread throughout the country, afflicting an estimated 15 million persons, with an estimated 1.5 million cases, or about half its population, in Rome alone, including Prime Minister Mariano Rumor.

Netherlands

An epidemic among the general population of the Netherlands began on 22 December 1968 and was reported as decreasing as of 15 February 1969. though it was not until mid-January 1969 that an outbreak of very high morbidity began among the general population, in contrast to most other European countries during the winter of 1968–1969. In total an estimated 3–4 million cases of influenza-like illness occurred during the first wave in Poland, with nearly 500,000 in Warsaw alone.

Spain was the first country to report the recurrence of the pandemic in Europe. The disease broke out again in early October 1969, evidently first in Madrid and later in the northern parts of the country, in particular Lugo and Navarre. The epidemic peaked during the month of November and was over by 27 December. On the whole, the disease was again generally mild, though pneumonia was a more frequent complication than usual. Isolated cases continued throughout the autumn, with a few localized outbreaks but no extensive spread in the general population. The first community outbreaks were reported towards the end of the year and became more frequent in January and February 1969. The virus indeed reappeared in October 1969 on the Isle of Lewis, Scotland, Soon thereafter, however, in early December, sickness benefit claims began to rise sharply. On 16 December, hospitals in London went on a yellow alert, stopping all but urgent admissions; on 27 December, the alert shifted to red, with only the most urgent flu cases being given beds. By the end of the year, emergency flu cases in the city had reached their highest level in seven years.

The epidemic interfered with English football, requiring several matches to be postponed. Three League matches set to be held on 20 December and three set for 3 January 1970 were called off due to sickness on the teams, while four FA Cup matches set for 3 January also had to be delayed.

Sickness benefit claims peaked at nearly 750,000 the first week of 1970, exceeding the peak of the 1957 pandemic. Consultation rates for influenza-like illness behaved similarly, peaking at 1,260 per 100,000 the last week of 1969 and the first week of 1970. Ultimately the wave subsided quickly, after just about six weeks, and was over by the end of January.

Cases increased again in late July 1969. After a brief decrease, cases again increased until the week ending 4 October, when they then declined. Serology tests at this time indicated pandemic influenza. However, some cases of illness diagnosed in late July and early August might actually have been Venezuelan equine encephalitis (VEE), which was at that time epidemic and epizootic. The later increase was very likely due to influenza, however. Several small outbreaks were later reported in November 1969, but incidence remained low.

Following the outbreak in Hong Kong and the recognition that it had been caused by a new variant of influenza, the CDC on 4 September revised its prediction for the 1968–1969 season. An extensive outbreak across the country was now more likely. It repeated more strongly its recommendation that existing vaccines go only to those at highest risk and recommended vaccinating or revaccinating this group once the monovalent vaccine specific to the new variant became available.

The first cases of the virus were reported in Atlanta on 2 September. The first was a Marine Corps major returning from Vietnam,

Isolated cases, mostly in those recently returning from the Far East, seeded the virus across the country throughout September. The first outbreaks in the civilian population occurred in late September and in October, and activity increased markedly throughout November, affecting 21 states by Thanksgiving. The Greater New York Hospital Association reported absenteeism of 15–20% among staff and urged its members to impose visitor restrictions to safeguard patients.

Institutions in many states dismissed their students early for the holidays. In New York and many other areas, holiday sales suffered mid-December, which affected retailers blamed on the flu epidemic (though inflation could have contributed to this as well). Economic activity was also hampered by high levels of industrial absenteeism. but whether the new variant was the cause of his illness was not made clear. He returned to the White House on 22 December. Vice President Humphrey was also reported to be ailing from the flu on the day Johnson's condition was revealed. On 23 December, it was reported that President-elect Nixon had been ill with the flu at his daughter's wedding the day before. Nixon later claimed that "the wedding cured the flu."

Peak influenza activity for most states most likely occurred in the latter half of December or early January, but the exact week was impossible to determine due to the holiday season. Activity declined throughout January. Excess pneumonia-influenza mortality passed the epidemic threshold during the first week of December and increased rapidly over the next month, peaking in the first half of January. It took until late March for mortality to return to normal levels. There was no second wave during this season.

Given the widespread epidemic levels of influenza A activity in 1968–1969, the CDC in June 1969 predicted little more than "sporadic cases" of influenza A in the 1969–1970 season. Influenza activity was indeed less than the preceding season, but there was "considerably more" than expected. The flu affected 48 states the following season but was widespread in only six, compared to 44 out of the 50 states in which activity was reported in 1968–1969.

In October 1969, the CDC, alongside Emory University, collaborated with the WHO to host an international conference on the novel influenza in Atlanta. A wide range of topics was discussed, including the origin and path of the pandemic, the experiences of individual countries, and effective control measures, such as vaccination.

South America

Argentina

Isolated cases of pandemic influenza were reported in Argentina in December 1968, though no large-scale outbreak occurred until mid-May 1969.

Pandemic influenza was first reported in West Africa: The Gambia saw a peak between November 1968 and March 1969, while Senegal saw increased activity between July and September 1969. North Africa potentially saw a peak in December 1968, based on data from Egypt. In East Africa, activity increased in Sudan from February 1969 through June 1969, while Kenya, Tanzania, and Uganda saw an increase from April 1969 to August 1969. The first cultures of the virus were provided to manufacturers in August by the Division of Biologics of the National Institutes of Health for preliminary study. A strain isolated in Japan was sent to the US and, after showing greater potential for vaccine production, was given to manufacturers on 9 September.

In 1968, American microbiologist Maurice Hilleman was head of the virus and vaccination research programs at the pharmaceutical firm Merck & Co., one of the licensed vaccine manufacturers in the US. Hilleman, as the director of the Department of Respiratory Diseases at the Army Medical School (now the Walter Reed Army Institute of Research), had foreseen the 1957 pandemic and kickstarted vaccine production then.

On 15 November, 66 days after the production strain became available, the first batch of 110,000 doses of vaccine was released, most of which went to the Armed Forces. President Johnson received "two types" of vaccine prior to his bout of flu in December, However, it was estimated that a "considerably higher" proportion of the recommended priority group of older and chronically ill persons received the pandemic vaccine than in 1957.

Following the epidemic in the US, leftover vaccine was made available for the southern hemisphere and parts of Europe where the main outbreak had not yet happened.

The same Japanese strain used for vaccine production in the US was immediately sent out to the seven manufacturing firms in Japan. It was soon decided a bivalent vaccine consisting of two parts the new variant and one part influenza B would be produced, in contrast to the US's use of monovalent vaccine. The objective was also set that enough vaccine to immunize about 12 million people would be produced by the end of October, with the hope of at least vaccinating children to guard against an epidemic developing out of schools. After some delay, the mass vaccination campaign was nearly completed before the end of the year.

In Denmark, the influenza department at the governmental Statens Serum Institut produced about 200,000 doses of pandemic vaccine during the winter of 1968–1969, incorporating a strain isolated in Stockholm. There were no particular difficulties in production, but yield was poor.

Millions of doses of vaccine were available in South Africa before its epidemic began at the end of March 1969, which afforded the opportunity to perform "limited studies" of its effectiveness.

By January 1969, vaccine production in Australia was underway at the Commonwealth Serum Laboratories (CSL), then a department of the federal government. The trivalent pandemic vaccine, composed of two influenza A strains and a B strain, was anticipated for release in early March ahead of the winter flu season. The inoculation consisted of a two-dose series, each given four weeks apart. A spokesman for the laboratories described the new virus as "the worst flu we have had" and called an epidemic that year "almost certain". In light of the situation, the Australian Pensioners Federation in early January wrote to Minister for Health Jim Forbes "demanding" that the vaccine be given free of charge to pensioners. In contrast to CSL's bolder predictions, Forbes described an outbreak that winter as "possible" but did not think it would "necessarily be serious or extensive". While the Department of Health reviewed the question of pandemic vaccine allocation in Australia, the government exported 1 million doses of its vaccine to Britain, already at the peak of its epidemic.

In early February, the epidemiology committee of Australia's National Health and Medical Research Council met in Melbourne to discuss the influenza threat and the best use of vaccine the coming winter. A "serious epidemic" was considered the "strongest possibility", and it was recommended to Forbes that older people, children, and pregnant women receive free immunization against the flu. However, the council advised against a mass vaccination campaign, citing the findings of its study which showed the unreliable protection against infection of the present vaccines, and considered it unwise to vaccinate healthy people while the limited supply could be better used to mitigate severe outcomes in at-risk groups.

On the last day of February, the Pharmaceutical Benefits Advisory Committee met to consider the question of making the pandemic vaccine a pharmaceutical benefit for pensioners.

Vaccination against the flu was recommended beginning 1 March, but issues surrounding availability of vaccine soon became apparent throughout the month. In response to Representative Gordon Scholes of Victoria, who had heard complaints from chemists unable to acquire vaccine, Forbes clarified that bulk orders from larger establishments would be met first. He relayed the expectation of the director of CSL that the present situation would be met once quantities of single doses became available in early April.

Representative Charles Jones of Newcastle later in the month questioned Forbes why his home city's order had not been filled. Forbes revealed the export of 1 million doses to Britain earlier in the year but assured that the order "did not delay, or in any way hinder, [the Commonwealth Serum Laboratories'] capacity to fill Australian orders" and that there would be enough supply to meet expected demand.

Despite these assurances from Forbes, the Director General of the Department of Health William Refshauge sent a letter on 9 April to all doctors in the country asking them not to vaccinate healthy people until at-risk groups in the community have been inoculated. Forbes reported meeting with the Commonwealth Serum Laboratories commission to discuss how to speed up distribution of vaccine. Two days later, the director of CSL, William R. Lane, dismissed criticism of the supply situation from the New South Wales branch of the Australian Medical Association as "a lot of nonsense". Contradicting the laboratories' more forceful marketing earlier in the year, he downplayed the likelihood of a serious epidemic but shared the expectation of 4 million doses distributed by the end of May, eight times as much as the average annual total distribution of 500,000 vaccine doses.

On 22 April, Forbes testified in the House of Representatives regarding the vaccine situation. He reported 2.5 million doses had been produced by this time since February. When asked by Representative Theo Nicholls of South Australia to consider importing vaccine to alleviate the present shortage, Forbes noted that the country had already imported the 150,000 doses available. He lamented CSL's recent subjection to a "good deal of abuse" regarding the "temporary shortages" around the country, repeating the comparison between the present production effort and the country's average annual distribution of only 500,000 doses. That same day, N. F. Keith, president of the Victorian branch of the Pharmacy Guild, called on CSL to explain the situation surrounding vaccine supply to the public, which was putting pressure on chemists due to the lack of vaccines.

On 25 April, it was reported that the Department of Health had reimported the remaining vaccine from the order of 1 million that the government had exported to Britain in January. After being sent to Britain, packaged there, and then sent back to Australia, it was sold to doctors at a markup of nearly 50 percent. Doctors criticized the Department and CSL's poor planning with respect to vaccine supply and the decision to export vaccine to Britain when it had already reached the peak of its flu season. They also blamed the shortage on an overreaction by the public, a response which they considered largely due to public statements made by CSL and health officials. It also denied any involvement in the commercial sales of vaccine, in response to reporting on price markups on the reimported vaccine, saying that all it did was authorize the reimportation and list the product as a pharmaceutical benefit. The government itself was paying the same for the reimported vaccine as it was for that being distributed by CSL.

Post-pandemic

The H3N2 virus ultimately displaced the previously circulating H2N2 virus, which first emerged in 1957.

It was during this period that the city of Coonoor, in India, experienced a "fairly extensive" outbreak, in July 1971. Samples of the virus responsible were collected but their significance was not immediately recognized. The virus did not immediately spread to other countries, or at least did not immediately cause outbreaks, but it was amid an epidemic in England in early 1972, fueled by more original strains, that a variant showing considerable antigenic drift was identified in one isolate tested out of over 700. It ultimately came to be designated A/England/42/72. It was soon recognized, by comparison with the strains isolated then, that this virus had been the one responsible for the epidemic in India.

The novel variant did not immediately spread after that outbreak, and circulating strains largely continued to resemble quite closely the original Hong Kong virus through April 1972. In May, however, at the onset of the flu season in the Southern Hemisphere, epidemics caused by the variant struck Malaysia, Singapore, and Australia, though South Africa and South America were unaffected. The die seemingly cast, the novel variant went on to cause widespread outbreaks in the Northern Hemisphere, by which point US press had dubbed the bug "London flu". It completely replaced the previous strains still resembling the original pandemic virus.

Soon after the initial outbreak in Hong Kong, the virus responsible was recognized to be antigenically distinct from the current influenza A strain in circulation (which at the time was called "A2") but was generally not considered an entirely new subtype. Analysis using the conventional techniques at the time revealed that it was indeed very different from older A2 viruses but also, at the same time, seemingly related to them, depending on one's reading of the data. Experiments involving newer methods of analysis soon identified another surface antigen, neuraminidase, in addition to hemagglutinin, which had already been recognized. It thus became clear that it was the hemagglutinin that had changed compared to older strains while the neuraminidase was identical. These findings, in part, prompted the World Health Organization in 1971 to revise its system of nomenclature for influenza viruses, taking into consideration both antigens. The novel virus was thereafter designated H3N2, indicating its partial similarity to H2N2 but also its antigenic distinction.

The H3N2 pandemic flu strain contained genes from a low-pathogenicity avian influenza virus.

The new subtype arose in pigs coinfected with avian and human viruses and was soon transferred to humans. Swine were considered the original "intermediate host" for influenza because they supported reassortment of divergent subtypes. However, other hosts appear capable of similar coinfection (such as many poultry species), and direct transmission of avian viruses to humans is possible. H1N1, associated with the 1918 flu pandemic, may have been transmitted directly from birds to humans.

Accumulated antibodies to the neuraminidase or internal proteins may have resulted in many fewer casualties than most other pandemics. However, cross-immunity within and between subtypes of influenza is poorly understood.

The basic reproduction number of the flu in this period was estimated at 1.80.

Mortality

The estimates of the total death toll due to Hong Kong flu (from its beginning in July 1968 until the outbreak faded during the winter of 1969–70) vary:

  • The World Health Organization and Encyclopaedia Britannica estimated the number of deaths due to Hong Kong flu to be between 1 and 4 million globally.

However, the death rate from the Hong Kong flu was lower than most other 20th-century pandemics. Fewer people died during this pandemic than in previous pandemics for several reasons: In the United States an estimated 28,100 to 56,300 people died over the winter of 1968–1969; most excess deaths were in those aged 65 and older.

References

  • Influenza Research Database – Database of influenza genomic sequences and related information.