[Note: This is a rewriting of my April 30 post.]
Herd immunity happens when most of a population are 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. In the meantime, the US will be slowly building herd immunity as more and more of us become infected. The vast majority of us will recover and cease being vectors of contagion (assuming immunity lasts until a vaccine is available).
Summer is coming, which is double-good news for outdoor enthusiasts: fun in the sun plus a much lower likelihood of getting infected by that awful virus. Before I explain why, check this out:
The above table documents the collapse of flu deaths during 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.
Here’s my prediction: young adults will flock to beach and parks. Some will violate social distancing guidelines. Despite lower infection rates in the summer, some of these free spirits will get infected by Covid-19. However, very few will become seriously ill. Even now, before the advent of hot weather, healthy 20-somethings have held up rather well under the viral onslaught, with a fairly low .03% COVID-19 death rate. And that rate will plummet to virtually nothing in the summer.
As these intrepid but infected souls recover and acquire antibodies, they will cease being vectors of contagion and thus play their part in the march to herd immunity. While such incomplete herd immunity will not stop a return of Covid-19 later in the year, 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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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