The “Hong Kong” or “Asian” influenza pandemic was caused by the H3N2 strain of the influenza A virus, mutated from H2N2. The first outbreak was recorded in Hong Kong in mid-1968; soldiers returning from the Vietnam War brought the infection to the United States. This infection, according to various estimates, claimed from 1 to 4 million lives, about 100 thousand people died in the United States.
The new study, led by scientists at Columbia University, has yet to be peer reviewed and published on the medRxiv preprint site. The authors hypothesized that the 1968 flu outbreak may have affected COVID-19 mortality in the same way that the Spanish flu pandemic did in reducing the incidence of tuberculosis. To test this hypothesis, scientists analyzed hospital admissions and deaths from Hong Kong flu in different age groups and compared the findings with information on the incidence and deaths from COVID-19 in the first half of 2020.
During the 1968 outbreak, about 40% of deaths occurred in patients under the age of 65. The researchers found that in areas hardest hit by the Hong Kong flu, mortality among people 60-69 years and older was lower than in other counties, by an average of 1-2%, including nursing homes. This age group also showed lower rates of complications and hospitalization compared to people of the same age who lived in childhood in areas where the impact of the pandemic was significantly lower. The scientists’ hypothesis was also confirmed by the fact that there was no trend towards a decrease in mortality in the age group of 40-59 years.
The authors of the study also did not find a connection between the decrease in mortality in this group of patients with the measures of the health authorities – for example, in these areas the mask regimen was not always followed, and the number of hospital beds was the same as in other districts at the beginning of the pandemic.
According to scientists, we are talking about a biological phenomenon, which is based on a generalized immune response received in early childhood or adolescence. It is also possible that the survivors of the 1968 pandemic had an innate defense against viruses that allowed them to survive.
“Thus, there could be some kind of selective influence in these places, where people who survived the 1968 pandemic were generally more prepared for the COVID-19 pandemic,” the study authors wrote.