‘Tis The Season To Be Careful

Everything from temperature differences to changes in social behaviour can affect the way a virus spreads, but what does the future hold for COVID-19?

- Jason Goodyer

What does the seasonality of a virus mean?

Dr. Mike Skinner, We are all familiar with viruses that routinely circulate, such as influenza, the common cold, and chickenpox, which is caused by the varicella-zoster virus (VZV). These viruses are endemic, which means they are regularly found among particular people, or in a certain area. Some of them, such as VZV, circulate all year round, while others, such as influenza, cause more infections in the winter. We say that these virus infections show ‘seasonality’. In some seasons, the peaks become so high that they are called epidemics, meaning a widespread occurrence of an infectious disease in a community at a particular time. COVID-19 is not yet endemic – having only emerged last year. However, it is an epidemic, in that it is still spreading rapidly. It can therefore only display seasonality if, or when, it becomes endemic.

Why do viruses show seasonality?

It’s still not fully established, but we believe a number of parameters come into play. Not all viruses show higher peaks of infection in the winter. For respiratory viruses, which do peak in winter, an important factor is the ability of the actual virus to resist environmental stresses. These include heat, humidity and UV light. Another important factor that affects seasonal virus transmission is our behavior. In winter, we tend to be more closely crowded together and create higher relative humidities in our homes because we remain inside and turn up the heating.

How does that apply to COVID-19?