As the number of coronavirus deaths continues to rise, some suggest that the warm spring weather approaching the northern hemisphere may slow down or stop the spread. US President Donald Trump echoed this: “Heat, generally speaking, kills this kind of virus.” But is that true?
The idea that spring is approaching comes from a comparison with the flu. In many ways COVID-19 is similar to the flu – both spread equally (respiratory secretions and contaminated surfaces) and both usually cause mild respiratory infections, which can develop into life-threatening pneumonia. But the transmission and intensity of COVID-19 are much higher than the flu. It is not clear whether COVID-19 transmission is affected by seasonal temperature variation. For the flu, the onset of spring starts to fall significantly in the number of cases that continue until cold temperatures return in the autumn. This seasonality of the flu is expected to result from the sensitivity of the virus to different environments and seasonal changes in the human immune system and in our behaviour patterns.
First, the flu virus looks better in cold, dry weather with ultraviolet light. Second, for many of us, shorter winter days can lead to lower levels of vitamin D and melatonin, which can affect our immune system function. Third, the more time we spend in the winter, with other people, inside and outside the home, the greater the chances of spreading the virus.
It is not clear what effect temperature and humidity have on coronavirus or its transmission. Other coronaviruses are seasonal, causing the common cold in winter.
The 2002-2003 Sars pandemic also began in the Northern Hemisphere winter and ended in July 2003, with a small revival of cases in the following winter. SARS cases peak during the warm month of May, and the end of the epidemic in July reflects the time needed for virus control rather than the impact of summer weather on virus spread. Also, the associated MERS coronavirus spreads mainly in hot countries.
Returning to comparison with the flu, the 2009-2010 influenza virus epidemic began in the spring, the spring thaw and the summer increased in strength, and reached its peak in the following winter. This suggests that in a pandemic, a large number of cases in many countries of the world could initiate continuous transmission of the virus throughout the summer, outweighing seasonal variations seen in minor infections. Although the WHO has not yet announced the COVID-19 pandemic, many experts believe we are fast approaching the pandemic.
The approaching warm weather will reduce viral transmission in the Northern Hemisphere (likely to increase transmission in the coming Southern Hemisphere winter), but the weather is less likely to end this growing epidemic.