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Will Summer Slow the Spread of COVID-19?

Will the Approaching Summer Climate Slow the Spread of the COVID-19?
    
    With summer being only a month away, many are hoping that the warmer climate will help slow the spread of coronavirus during the U.S.'s reopening phases. In support of this idea, some studies of similar viruses have indicated that cold and dry conditions increase the transmission of the virus. Among these viruses, influenza and respiratory syncytial virus have both shown an increase in transmission during times of low specific humidity and a reduction of cases in the summertime. However, not all viruses behave the same. Other infections such as enteroviruses, show peaking in the summer season. We do not know how COVID-19 will behave in response to changes in climate.

    On May 18, 2020, Science Magazine published an article predicting the effects of climate on COVID-19. Their study focused on two coronaviruses (HCoV-HKU1 and HCoV-OC43) and influenza, which are currently circulating in human populations. Although there is no direct data to study the sensitivity of transmission within COVID-19, these two coronaviruses share the same betacoronavirus genus as SARS-CoV-2 and already have the necessary data. Researchers used simulation methods to predict the outcomes of the three viral outbreaks. The results from the coronaviruses and influenza will help us understand the relationship between COVID-19 transmission and climate. 

The Results

    One figure from the paper (see below 1A) shows the relationship between specific humidity and transmission of influenza and the two coronaviruses, HKU1 and OC43. In all cases, transmission rate increases as specific humidity declines. However, influenza and HKU1 transmission rates have a stronger dependency on specific humidity compared to OC43. Even though HKU1 and OC43 share the same genus, their results differ significantly. With that in mind, it is hard to predict how COVID-19 will follow suit. 

*R0 (y-axis) is the average number of people who will contract the disease from one person

    The next question to be answered by the researchers was whether various climates from regions around the world would affect transmission rates. The authors simulated a pandemic invasion in nine cities, each with different climates. The nine cities were evenly divided in the Northern, Southern and Tropical Hemispheres. The three cities in the Northern Hemisphere all had a similar pandemic size despite the different climates in the cities (New York, London and New Delhi). Influenza and HKU1 both had a lower intensity pandemic in the tropical locations compared to the Northern Hemisphere. Meanwhile, OC43 had the least dependence on climate, as shown in the figure (C-E) below (from the same paper). The peak of the pandemic occurred between February and April (Figure B).

    
    The key result of the simulation showed that in all three scenarios, climate only became an influential factor when the majority of the human population was immune to the virus. According to Rachel Baker, the first author of the paper and a research associate at Princeton, "The more that immunity builds up, the more we expect the sensitivity to climate to increase".

    Another simulation accounted for the impact of control measures including social distancing. The results indicated that the longer the control measures take place, the more sensitive the virus will become to climate in the future.

In Conclusion

    The results indicate that climate is unlikely to play a significant role in slowing the spread of COVID-19 during its early pandemic. Although the HKU1 scenarios show a sustained and lower intensity pandemic in warmer climates, the OC43 scenario with higher intensity pandemics despite tropical climates, is just as likely to occur. COVID-19's response to the weather is still unknown but it's unlikely that warmer conditions will not completely halt transmission based on results from other viruses. Population immunity is still the most effective way to lower the pandemic intensity. Therefore, it's highly unlikely that the current pandemic will automatically go away with the approaching summer climate. 


Sources:

  1. Baker, Rachel E., et al. “Susceptible Supply Limits the Role of Climate in the Early SARS-CoV-2 Pandemic.” Science, American Association for the Advancement of Science, 18 May 2020, science.sciencemag.org/content/early/2020/05/15/science.abc2535.

  2. Princeton. “Local Climate Unlikely to Drive the Early COVID-19 Pandemic.” ScienceBlog.com, 19 May 2020, scienceblog.com/516439/local-climate-unlikely-to-drive-the-early-covid-19-pandemic/.

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