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Sudden drop in air temperature appears to ‘kickstart’ flu season, study finds

REUTERS/Andrew Kelly
The number of people who contracted influenza A jumped significantly about one week after temperatures fell below freezing for the first time.

A recent study from Sweden suggests that the start of the annual flu season can be predicted by watching weather reports.

According to the study’s findings, the flu season appears to take off about one week after a sustained cold period, when temperatures dip below freezing and humidity is low.

“We believe that this sudden drop in temperature contributes to ‘kickstart’ the epidemic. Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected,” said Nicklas Sundell, the study’s lead author and an infectious disease specialist at Sweden’s Sahlgrenska University Hospital, in a released statement.

Three years of data

For the study, Sundell and his colleagues relied on more than 20,000 nasal swabs collected from individuals who sought medical care in Gothenburg, Sweden, over the course of three years (October 2010 to July 2013). The researchers first determined which samples contained viruses, including the human influenza A virus, one of two viruses most responsible for seasonal flu epidemics each winter. They then compared those findings with weather data in Gothenburg, including daily temperatures and humidity.

They found a consistent pattern in each of the three years: The number of people who contracted influenza A jumped significantly about one week after temperatures fell below freezing (32 degrees Fahrenheit) for the first time.

And once the outbreaks began, they continued, even if air temperatures rose again.

Interestingly, influenza A was not the only virus that followed this weather pattern. Outbreaks of other common viruses that cause respiratory tract infections, such as respiratory syncytial (RS) virus and coronavirus, were also found to start about a week after the first cold snap of the year.

Rhinoviruses, which cause the common cold, did not appear to be affected, however, by a sudden downward shift in temperature. So, although you’re more likely to “catch a cold” in the winter and spring, you can also do so at other times of the year. 

Not the only factors

As Sundell and his colleagues note, their findings support the hypothesis that air particles containing virus-laden respiratory mucus are better able to spread in cold, dry weather. The dry air absorbs the moisture and also reduces the size of the particles, which helps keep them airborne.

The researchers suggest that in northern latitudes like Sweden’s (and, presumably, Minnesota’s), the onset of a cold snap may be more important than indoor crowding for spreading the flu.

Still, cold weather doesn’t tell the whole story about seasonal flu outbreaks. “Cold and dry weather and small aerosol particles are important [prerequisites]  for the flu epidemic to take off,” said Sundell. “But cold weather isn’t the only contributing factor. The virus has to be present among the population and there have to be enough people susceptible to the infection.”

Limitations and implications

This study has several limitations. Most obvious is the fact that it involved a rather homogeneous group of people living in one city. Whether the findings would be similar in other cities — particularly cities at different latitudes — is unknown.

Still, the findings are intriguing — and may prove helpful for limiting the spread of seasonal influenza, which annually sends, on average, more than 200,000 Americans to the hospital and claims 3,000 to 49,000 lives, depending on the type of virus circulating in any particular year. 

“If you can predict the start of the annual epidemics of the flu and other respiratory viruses, you can use this knowledge to promote campaigns for the flu vaccine and prepare emergency wards and hospital staff in advance for an increased number of patients seeking care,” said Sundell.

“The recommendations are the same as previous years: vaccination of risk groups, cough and sneeze into your elbow, and remember to wash your hands,” he added.

FMI: The study was published earlier this month in the Journal of Clinical Virology, but, unfortunately, the full paper is behind a paywall. Also, it’s not too late to get a flu vaccine this year. To find a clinic offering vaccines near year, go to the Minnesota Department of Health’s “Vaccine Clinic Look-Up” website.

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