In the U.S. and the rest of the Northern Hemisphere, the flu is usually considered a wintertime illness. However, some of the most significant influenza pandemics of the past century have had initial peaks in the summer. Among them is the Spanish flu pandemic of 1918-1919 (the “mother of all” influenza pandemics), which emerged in the U.S. in March 1918 and spread across Europe in May and June. The more recent swine flu (H1N1) pandemic in 2009 also began in the spring and reached pandemic levels by June. Another notable pandemic is the Hong Kong flu (H3N2), which emerged in July 1968 and peaked after only two weeks. Because these influenza pandemics did not emerge during the typical “flu season,” they highlight the importance of maintaining flu surveillance during the off-season.
The difference between a regular flu season and a pandemic is that influenza pandemics exhibit explosive transmission and high morbidity but low mortality (meaning that although many people may die, the deaths only represent a small proportion of the much larger total who fall ill). All known influenza pandemics in human history have been caused by different subtypes of the influenza A virus (the other virus types are influenza B and C). The influenza A subtypes that have historically caused human disease are three HA subtypes (H1, H2, and H3) and two NA subtypes (N1 and N2). For example, the Spanish flu of 1918 was H1N1, the Asian flu of 1957 was H2N2, and the Hong Kong flu of 1968 was H3N2. More recently, the H5, H7, and H9 subtypes of HA have been discovered to cause human disease.
Avian influenza A (H7N9) had been found only in birds until March 2013, when human cases were discovered in China. Since then, there have been 429 H7N9 cases including at least 100 deaths. Human to human transmission of the disease remains limited, and most human cases result from contact with infected poultry or contaminated environments. Although it emerged in the spring of 2013, this strain of influenza did not exhibit an initial summertime peak and has not reached pandemic levels. However, an increase in infections since October 2013 may indicate a seasonal pattern similar to that of highly pathogenic influenza A (H5N1), in which cases are most common in winter months. H5N1 has infected 665 people including 392 deaths since its emergence in humans in 2003.
Although the H7N9 and H5N1 avian influenza strains have not caused pandemics or infected significant numbers of people during the summer, the risk of a new influenza strain emerging and causing a summertime pandemic is not too far-fetched—just look at the history. Influenza outbreaks are unpredictable, and two or more influenza strains can combine to form a new strain at any time, whether it's the flu season or the off-season.
If you are interested in participating in a year-round citizen science influenza surveillance project, check out Flu Near You (flunearyou,org). Its year-round surveillance can provide an early warning signal if another pandemic emerges.