In This Article
- Potential determinants of the seasonality of endemic respiratory viruses include changes in host behavior, immunity, temperature and humidity
- Pandemics often do not adhere to typical viral seasonality
- It's unknown whether transmission of SARS-CoV-2 will be different during summer in the northern hemisphere
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There's is no way to know whether transmission of the novel coronavirus, SARS-CoV-2, will be different this summer. There are reasons to be hopeful, but pandemics often do not adhere to viral seasonality, according to a fast literature update posted by Jason W. Griffith, MD, PhD, critical care specialist in the Division of Pulmonary and Critical Medicine at Massachusetts General Hospital, on April 17, 2020.
Potential Factors in Seasonality of Respiratory Infections
Host behavior: During the 2009 pandemic of H1N1 influenza, school closures resulted in 25% to 30% reductions in virus transmission in Mexico, Hong Kong and Europe. Another wave resurged in Mexico after school resumed in the fall.
Host defense: Cold, dry air is likely to impair mucociliary clearance, and it is known to reduce immune responses to rhinovirus.
Temperature and humidity: In animal models, influenza transmission decreased with decreasing vapor pressure, which in turn related to the duration of viable virus in aerosols. In the U.S., the onset of influenza-related mortality is associated with low absolute humidity during the previous weeks.
Lessons from Influenza Pandemics
All recent influenza pandemics (1918, 1957, 1968 and 2009) arose in the northern hemisphere in the spring or summer. In three of them, a second or third more geographically dispersed wave developed the following fall or winter.
Pandemic viruses appear to have different properties than endemic viruses, likely due to people's lack of pre-existing immunity.
What About Coronaviruses?
In a U.K. screening program in 2006–2009, four endemic circulating coronaviruses demonstrated marked winter seasonality. Moreover, both SARS-CoV, the coronavirus that caused the SARS epidemic of 2002–2004, and SARS-CoV-2 emerged during winter months.
Accordingly, some have proposed that summer will reduce the incidence of SARS-CoV-2 infections. At this point, the answer is unknown. Intensive public health interventions, not weather changes, stopped SARS. The hopes placed on warmer temperatures should not replace physical distancing, mask-wearing and other containment measures.
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