A fascinating new study suggests that the transmission of COVID-19 may fluctuate seasonally, and that the spread may slow down as temperatures rise. On the flip side, we may see a resurgence of the virus heading into fall and winter.
Key Study Findings
- Warmer temperatures by 1°C (1.8°F) reduced the transmission rate of COVID-19 by 13%.
- Changes in temperature from March to July 2020 may lead to a 43% decrease in COVID-19 transmission in Northern Hemisphere countries like the US.
- Southern Hemisphere countries may see a 71% increase in transmission during the same period.
- By winter 2021, the US may see a resurgence of COVID-19 with a 59% increase in transmission from the current (March 2020) rate.
- Due to the potential seasonality of COVID-19, northern countries like the US may have a window of opportunity over the summer months to recover and to prepare for a possible resurgence in winter.
The study, posted yesterday as a preprint (ahead of being accepted for publication), found that higher temperatures decreased the number of new COVID-19 cases in a community. The authors looked at data from 134 countries based on 166,686 COVID-19 cases diagnosed between January 22 and March 15, 2020. They found an average decrease in transmission rate (the number of new cases) across all countries was 13% for every 1°C (1.8°F) increase in temperature. This drop in transmission can have a major impact on total case numbers, particularity in areas currently suffering from a high rate of spread, like New York and other major US cities.
By modeling these findings out further, using projected temperature changes from April 2020 through January 2021, the authors predicted a 43% decrease in COVID-19 transmission across the Northern Hemisphere by July 2020 compared to today. Because the Southern Hemisphere is heading into its winter months, the authors project a 71% increase in new cases over the next 3-4 months – an alarming finding that should be noted by health officials in Southern Hemisphere countries.
As a warning to the US and other countries of the Northern Hemisphere, the study also reported a possible resurgence of COVID-19 by winter, with a 59% higher transmission rate in January 2021 compared to the rate today. This means that while we may experience a period of recovery over the summer months, we should also take that time to prepare for the next potential wave of cases. We may continue to see this seasonal ebb and flow of cases until we either reach a critical threshold for herd (community) immunity, or we have a COVID-19 vaccine, which most believe is 12-18 months away at best.
So why does temperature have anything to do with how a virus spreads? There are a number of possible mechanisms including temperature playing a role in attenuating (weakening) the virus itself. Other possibilities are that warmer temperatures encourage outdoor activities, which naturally results in more social distancing versus when people are confined to indoor spaces and are in close proximity to one another. Also people are generally healthier in the warmer months, when other seasonal illnesses are less of a threat.
This research provides clues as to whether we can expect the transmission of COVID-19 to shift as temperatures shift around the globe. However, these projections are based on early case numbers, most of which were concentrated in Europe and Asia. The authors did account for differences across countries (and regions within countries) in population density and health system capacity including the amount of COVID-19 testing, and still saw the relationship with temperature.
We can expect to learn a great deal more about the seasonality of COVID-19 as more cases are identified and as temperatures change. The authors suspect we may eventually see an even stronger impact of temperature on transmission. We may not know for sure until it happens. Until then, countries should continue to respond to the COVID-19 threat at hand. We can hope for a summer reprieve, but must also consider a possible resurgence come cooler months. Countries heading into the winter now should be aware of the potential for an increase in transmission of COVID-19.
Study: Tamma Carletonk and Kyle C. Meng. Causal empirical estimates suggest COVID-19 transmission rates are highly seasonal. medRxiv preprint doi: https://doi.org/10.1101/2020.03.26.20044420.
Accessed March 30, 2020.