Early in the pandemic, experts debated whether COVID-19 would behave like a seasonal virus. A new study shows that temperature and population density are the main factors that determine how easily the virus spreads — but only when there are no mobility restricting policies in place.

The results suggest that colder temperatures in autumn and winter may increase viral spread — especially if policy interventions have been rolled back and the public begins easing up on mitigation measures such as mask wearing, wrote first author Tom Smith, Ph.D., of Imperial College London.

That’s because temperature changes have a much smaller effect on transmission than policy interventions and subsequent human behavior changes do, Smith added. “So while people remain unvaccinated, governments mustn’t drop policies like lockdowns and social distancing just because a seasonal change means the weather is warming up,” he cautioned.

The study is the first to include environmental data in an epidemiological model, according to Smith and colleagues. But quantifying the effects of environmental factors including temperature, humidity and UV radiation on SARS-CoV-2 transmission has been difficult, they said. Unlike influenza virus transmission, which is known to be slowed by high temperatures and low humidity, transmission of the novel coronavirus has been heavily influenced by many other factors, including population density, herd immunity, and behavior, along with unprecedented non-pharmaceutical control measures.

The study was published Wednesday in the Proceedings of the National Academy of Sciences.