Flu Season Rages on as Researchers Learn More on How Climate Affects the Virus

As the 2016-2017 flu season continues to see more widespread illness across the United States, a recent study has highlighted how winter weather may trigger seasonal flu epidemics.

Flu season typically begins in October and can last until as late as May, and researchers have long tried to understand the correlation between winter weather and influenza outbreaks. In a new study published in the Journal of Clinical Virology, researchers from the University of Gothenburg in Sweden investigated the link between declines in average temperature and the onset of influenza A. They found that annual epidemics of influenza and other respiratory infections tend to begin within the week following sudden drops in outdoor temperature and vapor pressure, showing the marked seasonality of many respiratory viruses. “We believe that this sudden drop in temperature contributes to ‘kickstart[ing]’ the epidemic,” explained study author Nicklas Sundell in a recent press release. “Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected.”

The Centers for Disease Control and Prevention (CDC) is reporting widespread influenza activity in 29 states, up from 21 states in the previous week. Massachusetts is one of the New England states experiencing a steady rise in flu cases, with state health officials noting that flu season in the region tends to peak in February or March. According to the Massachusetts Department of Public Health’s weekly influenza update, there have been 823 laboratory-confirmed flu cases so far this season, a large increase compared with the 95 total confirmed cases last flu season. State health officials note that not all flu cases receive lab confirmations, and the actual number of flu illnesses in Massachusetts so far this season may even be higher than official tallies show. As with the rest of the country, the influenza viruses causing illness in Massachusetts are a strong match to this season’s flu vaccine, which is recommended for everyone 6 months and older.

While the CDC typically recommends that individuals receive the influenza vaccine by the start of flu season in October—noting that vaccination can continue throughout the period when flu viruses circulate—new questions have been posed regarding if that is, in fact, the best time to receive the shot. Vaccine efficacy has come under fire in recent years, as receiving a flu shot does not necessarily guarantee that an individual will be protected from circulating viruses. In a recent study published in the journal Clinical Infectious Diseases, researchers from the CDC found that the effectiveness of the flu vaccine may actually wane over the course of the flu season. The study looked at influenza vaccine effectiveness from the 2011-2012 season through the 2014-2015 season, comparing confirmed influenza infection to vaccination status and the time between vaccination and the onset of illness. The researchers found that maximum vaccine effectiveness occurred in the time shortly after vaccination, with effectiveness declining by about 7% per month for influenza A (H3N2) and by 6% to 11% per month for influenza A (H1N1) and influenza B viruses. Furthermore, the study found that the decline in vaccine effectiveness was more pronounced in those who had received a flu shot prior to the start of the season.

Despite the new findings, health officials still stress that even with its imperfections, the influenza vaccine is the best way to prevent illness and as the virus continues to hit the United States, it’s not too late to receive a vaccine for this season.