At first glance, it seems very plausible that SARS-CoV-2 – the virus that causes COVID-19 – could behave seasonally, being more common in winter and less common in summer. The four other coronaviruses that commonly circulate in humans act like that. We have also seen COVID-19 cases, hospitalizations, and deaths Great over the winter in the UK and other countries, suggesting a seasonal effect.
Some correlation between virus transmission and the time of year can be expected. Many human behaviors are seasonal. In summer we spend more time outdoors, where there is a risk of infection much lessand we will likely lead a more active lifestyle, which is possible increase the ability of the body Resist Infections. We will likely benefit from increased exposure to the sun as well increases vitamin D levels and can thus strengthen our immune system.
There is also evidence that the ultraviolet (UV) radiation in sunlight reduced how long The virus can survive on surfaces. It is also possible that moisture and temperature will affect transmission. When combined, these factors are likely to affect the spread of the virus.
But how significant is this effect? And what are the implications for COVID-19 control as we approach the warmer months, as well as the potential for another winter resurgence? Since the existing studies had led to inconclusive results about whether and how the seasons affect SARS-CoV-2, my colleagues and I set about whether we could find even more conclusive answers to these questions.
Assessment of the effects of the climate
Epidemiologists use something like that Reproduction number or R, to describe the growth of an epidemic – the higher the R-number, the faster the spread. At the beginning of an epidemic, their growth is not affected by someone being exposed to the disease and developing immunity. Hence, it will expand exponentially. At this point, the R number describing this spread is referred to as R₀.
Using data from outbreaks around the world, our new research determined R₀ for COVID-19 in 359 major cities. Each city included in our study had a population of over 500,000 and had experienced a significant COVID-19 outbreak in 2020.
We focused on large cities (rather than countries or smaller populations) because it allowed us to study outbreaks large enough and geographically diverse enough to allow useful comparisons. By comparing the cities’ outbreak data with information about their demographics, climate and infection control measures, we were then able to determine if any of these factors explain the rate of spread of the virus.
We found that increased UV radiation corresponded to a decrease in the rate of spread of the virus. On average, R₀ decreased by 0.05 for every ten kilojoules per square meter (kJ / m²) increase in daily UV radiation (cities in our data set ranged from 30 kJ / m² to around 130 kJ / m² UV per day).
Since UV radiation is higher in summer, our results suggest that there is indeed some seasonal effect on transmission. It is important to note, however, that this correlation does not necessarily mean that UV radiation is the cause of this decrease in transmission, as UV radiation can correlate with other causal factors.
For example, the higher the UV radiation in a city, the hotter it is. We have not found a separate statistically significant relationship between R₀ and temperature or humidity on a global scale, but we cannot rule out such relationships.
The relationship between the spread of the virus and temperature or humidity may have been masked by many other factors affecting R., as well as the strong correlation between UV radiation and temperature. Indeed there are some weak evidence an association between virus spread and temperature in other studies.
What does that mean?
While the effect of UV radiation we observed was statistically significant, it was relatively small compared to other factors. City demographics such as the size and amount of air pollution (a potential measure of industrialization and population congestion) and public health measures accounted for a larger proportion of the observed variations in R₀ values.
Government interventions accounted for about four times the explainable variation in R₀ compared to UV. This is in our control. In the near future, potential further waves of the pandemic will be largely determined by controls dictated by governments rather than the weather. Add to this the effects of the COVID-19 vaccines that are currently being rolled out.
In the long term, the question remains whether COVID-19 will become a seasonal endemic infection similar to influenza and other coronaviruses. Our research has found evidence of small seasonal drivers that may produce this type of variation if COVID-19 is likely to stabilize as an endemic infectious disease.
However, predicting this behavior for a system as complex as the world is difficult, and as we move out of the initial epidemic phase, the longer-term behavior of COVID-19 transmission will likely depend on many other factors. These likely include the level and duration of immunity acquired by infected individuals, as well as the effectiveness and duration of protection from current and future vaccines and the development of new variants of the virus.