Study: Flu most contagious in freezing temperatures, Covid at +10 degrees

When the Covid-19 epidemic first started, it was widely thought that it might spread similarly to influenza, but new research found differences in how the viruses spread.

In 2004-2005, the University of Oulu studied influenza infections among conscripts. It turned out that particularly rapid decreases in temperature increased new cases of influenza. The virus was most contagious in temperatures just above or below zero degrees Celsius. Photo of new conscripts in Santahamina on March 3, 2021. Image: Jyrki Lyytikkä / Yle

When coronavirus first appeared, it was not thought that meteorological or temperature differences had a major effect on the virus' ability to spread.

However, a newly-published international study has shown that the weather can affect infection rates in several ways. But it also found little evidence that meteorological conditions particularly influenced local spreads of Covid-19, concluding that people's behaviour and government interventions had a larger impact on infection rates.

The extensive study collected (siirryt toiseen palveluun) data from more than 400 locations across 26 countries — including Finland — and from all of the globe's climatic zones.

The regions were examined at periods when resistance levels to the virus were still low and Covid vaccinations had still not yet rolled out.

Although weather conditions had little effect on the overall spread of the virus, the researchers noted something when comparing Covid's R0 number versus the local temperature. The R0, or reproduction number, measures how contagious a disease is.

The researchers found that the R0 began to rise as temperatures reached +10 degrees Celsius and then started to taper off as the mercury hit +20 degrees. The virus was found to spread most in environments featuring temperatures between eight to 18 degrees.

10 degrees Celsius

According to University of Oulu public health professor Jouni Jaakkola, coronavirus is most contagious at a temperature of 10 degrees.

"Infectivity decreases as the temperature rises to 20 degrees and it also decreases as the air cools down towards zero degrees," Jaakkola said.

The study found, however, that interventions like government restrictions had a six-fold greater impact on the spread of coronavirus compared to the relative temperature.

But Jaakkola noted that weather conditions could play an increasing role in the spread of the virus as the pandemic subsides.

Story continues after photo

Jouni Jaakkola Image: Timo Nykyri / Yle

"My own opinion is that in the early stages of the coronavirus pandemic, there were a number of other factors that had a strong influence on how it spread. These include the inventions and the extent to which populations have been immunised, initially through infection and then through vaccinations," the professor explained.

The effects that temperatures have on the spread of the virus are a combination of many factors. Temperature affects the virus organism itself, but also affects people's resistance to it as well as impacting human behaviours.

Influenza differences

Unlike the coronavirus, influenza reacts differently to the relative temperature. Previous studies have linked a drop in outdoor temperatures to an increase in influenza cases. In other words, the temperature is not as relevant to the spread of influenza, but when it gets colder, infections increase.

The new study found that the risk of influenza infection was highest in areas that featured temperatures ranging between -5 to +5 degrees Celsius. Initially, the researchers assumed this would also be the case for coronavirus.

"Influenza infections follow seasonal variations and typically occur during the winter season. This has continued for at least one hundred, if not hundreds of years, and some balance has been reached here. We assumed that if there was a similarity [between coronavirus and influenza] then the epidemic should decrease in the spring and summer in the same way as the flu. This was not clearly confirmed," Jaakkola said.

Research on this topic will continue in Finland, according to the professor. Researchers at the University of Oulu are planning to examine data from the two first years of the Covid-19 era, including more study on the possible significance that seasonal variations may have on the virus.