The Seasons of the Flu

A new study published through the Public Library of Science this past Tuesday suggests that China should stagger the start dates of its influenza vaccination programs depending on region, due to differences in seasonal peaks of infection. According to the study, northern China’s influenza epidemic peak typically falls between January and February, thus requiring vaccinations in October, while in the south infection peaked between April and June, calling for vaccinations to begin in February. Central China showed a much more complex structure of infection and researchers claim more work needs to be done before a competent strategy can be developed.

Influenza A virus (Source:

The study, led by researchers from the Fogarty International Center with the National Institutes of Health, examined weekly influenza infection records compiled from reports taken by sentinel hospitals all around China. Their data represented influenza infection for 30 Chinese provinces from 2005-2011, and took into account a wide variety of other factors, including socio-economic conditions, regional climate, and population demographics.

A map of the survey sites for the study. Triangles indicate surveillance from October through March, circles indicate year round. (Source:

The group also found that only influenza A infection was dependent upon season, while influenza B showed a fairly constant preference for infection during the colder months all across China. Influenza A differs morphologically and functionally from B, in that it infects humans along with other mammals and some birds and is known to cause seasonal epidemics, while influenza B is not thought to cause epidemics and only infects humans and seals. Yes. Seals.

While China introduced its vaccination program back in 1998, at this point only about 2 percent of their impressive 1.34 billion citizen population gets vaccinated, according to the NIH press release regarding the study. As such, they see about 11-18 excess deaths per 100,000 people in pandemic or inter-pandemic seasons. According to the paper, influenza epidemics claim about a half million people worldwide every year.

While this new study certainly leaves some questions to be answered (like what should be done with that problematic central region, found to have two peaks stretching from January to February and June to August), it does help in starting to lay out a plan for how China could design a national vaccination program. With more work and some solid research, China could help to alleviate what, at this point, shouldn’t really be such a serious problem.

Original Article:

NIH Press Release:


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