Herd immunity happens when most of a population is immune to an infectious disease. This provides indirect protection to those who are not immune to the disease. Herd immunity is achieved either through mass vaccination, mass infection, or a combination of both. In the case of COVID-19, epidemiologists are guessing around 70% of the population will need immunity for the US to achieve herd immunity. Ideally, we would wait for a vaccine to actively pursue herd immunity, but a vaccine might not be ready for another year. The US can’t shut down its economy that long. What to do?
We obviously have to ease the lockdown some time soon to avoid economic collapse. But should everyone continue to maintain strict social distancing rules in public settings? People in the vulnerable groups (e.g.., old, obese, immunocompromised, or with an underlying medical condition) obviously should, and some of these individuals should probably continue to shelter-in-place. But how about the rest of the population? Such as healthy 20-somethings, with their .03% COVID-19 deathrate. And that’s their mortality rate at this time. COVID-19 mortality rates are going down throughout the US and will be much, much lower by June. Check this out:.
The above table documents the collapse of flu deaths every summer over a 30-year period. And there is every reason to expect a similar reduction in COVID-19 mortality this summer. It’s a weather effect. Specifically, cold, dry winter air increases both the transmission and severity of viral respiratory infections by:
Affecting the properties of the virus surface proteins and membrane.
Making it easier for airborne viral particles to travel.
Reducing the functionality of teeny hair-like structures called cilia from expelling viral particles from the body’s airways.
Impairing cell repair in the lungs exposed to viral infections.
Reducing immune system efficiency.
So if summer weather reduces viral spread and severity, why not use that time to build partial herd immunity by encouraging healthy young adults to get out in the sun and thus be exposed to COVID-19 at a time when it’s less lethal than the flu in winter? Partial herd immunity will not stop a return of COVID-19 later in the year, but it may slow its spread when the weather cools*. And slowing the spread will continue to save lives as we wait for a vaccine to provide full herd immunity.
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* Of course, any loosening of social distance rules would need to be monitored closely with the understanding that rules will be reinstated if infection or mortality rates increase.
References:
Doshi P. Trends in recorded influenza mortality: United States, 1900-2004. Am J Public Health. 2008;98(5):939‐945.
“How humidity may affect COVID-19 outcome” by Ana Sandoiu and fact checked by Catherine Carver, MPH Medical News Today
Moriyama, M., W. J. Hugentobler, et al. (2020). "Seasonality of Respiratory Viral Infections." Annual Review of Virology. https://doi.org/10.1146/annurev-virology-012420-022445
“What is Herd Immunity and How Can We Achieve It With COVID-19?” by Gypsyamber D’Souza and David Dowdy Johns Hopkins Bloomberg School of Public Health
“What Is Herd Immunity and Could It Help Prevent COVID-19?” by Noreen Iftikhar, MD/Healthline