Do you have monthy data from 1968 that shows the deaths were evenly distributed over the entire year or should we just assume their flu season was super unusual and never abated?
https://en.wikipedia.org/wiki/Hong_Kong_flu
[The last sentences are the key. It was spread over 3+ years, from 1968-70.]
History
The recorded instance of the outbreak appeared on 13 July 1968 in Hong Kong. (There is a possibility that this outbreak actually began in mainland China before spreading to Hong Kong, but this is unconfirmed.[5]) By the end of July 1968, extensive outbreaks were reported in Vietnam and Singapore. Despite the lethality of the 1957 Asian Flu in China, little improvement had been made regarding the handling of such epidemics. The Times newspaper was the first source to sound alarm regarding this new possible pandemic.
By September 1968, the flu had reached India, the Philippines, northern Australia, and Europe. That same month, the virus entered California, carried by returning troops from the Vietnam War, but did not become widespread in the United States until December 1968. It reached Japan, Africa, and South America by 1969.[6]
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 due to insufficient undertakers. In total, East and West Germany registered 60,000 estimated deaths. In some areas of France, half the workforce was bedridden, and manufacturing suffered large disruptions due to absenteeism. The British postal and train services were also severely disrupted.[7]
The outbreak in Hong Kong, where population density was greater than 6,000 people per square kilometre, reached maximum intensity in two weeks; it lasted six months in total from July to December 1968. Worldwide deaths from this virus peaked in December 1968 and January 1969. By that time, public health warnings[8] and virus descriptions[9] were widely issued in the scientific and medical journals.
In comparison to other pandemics of the 20th century, the Hong Kong flu yielded a low death rate.[6]
The H3N2 virus returned during the following 1969/1970 flu season, resulting in a second, deadlier wave of deaths.[10] It remains in circulation today as a strain of the seasonal flu.[10]
Clinical data
Flu symptoms typically lasted four to five days but some persisted for up to two weeks.[6]
Virology
The Hong Kong flu was the first known outbreak of the H3N2 strain, though there is serologic evidence of H3N1 infections in the late 19th century. The virus was isolated in Queen Mary Hospital.
Both the H2N2 and H3N2 pandemic flu strains contained genes from avian influenza viruses. The new subtypes arose in pigs coinfected with avian and human viruses and were 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 (for example, 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.[11]
The Hong Kong flu strain shared internal genes and the neuraminidase with the 1957 Asian Flu (H2N2). Accumulated antibodies to the neuraminidase or internal proteins may have resulted in many fewer casualties than most pandemics. However, cross-immunity within and between subtypes of influenza is poorly understood.
Mortality
The United States Centers for Disease Control and Prevention (CDC) estimated that in total, the virus killed one million people worldwide,[12] from its beginning in July 1968 until the outbreak faded during the winter of 1969-70.[13] The CDC estimated fewer than 100,000 people died in the U.S; most excess deaths were in those 65 and older.[12] However, fewer people died during this pandemic than in previous pandemics for several reasons:[12]
Some immunity against the N2 flu virus may have been retained in populations struck by the Asian Flu strains which had been circulating since 1957;
The pandemic did not gain momentum until near the winter school holidays, thus limiting the infection spreading;
Improved medical care gave vital support to the very ill;
The availability of antibiotics that were more effective against secondary bacterial infections.
For this pandemic, there were two geographically distinct mortality patterns. In North America (the United States and Canada), the first pandemic season (1968/69) was more severe than the second (1969/70). In the “smoldering” pattern seen in Europe and Asia (England, France, Japan, and Australia), the second pandemic season was two to five times more severe than the first.[14]