28 May 2021

STAT: “How the Covid pandemic ends: Scientists look to the past to see the future”

How did those pandemics end? The viruses didn’t go away; a descendent of the Spanish flu virus, the modern H1N1, circulates to this day, as does H3N2. Humans didn’t develop herd immunity to them, either. That’s a phenomenon by which a pathogen stops spreading because so many people are protected against it, because they’ve already been infected or vaccinated.

Instead, the viruses that caused these pandemics underwent a transition. Or more to the point, we did. Our immune systems learned enough about them to fend off the deadliest manifestations of infection, at least most of the time. Humans and viruses reached an immunological détente. Instead of causing tsunamis of devastating illness, over time the viruses came to trigger small surges of milder illness. Pandemic flu became seasonal flu.

That immune system training will likely turn future Covid-19 infections into the equivalent of a cold, the authors concluded. Over time, as a degree of protection becomes more standard in adults, the people who will most commonly catch Covid will be young kids, in whom infections even now are rarely serious. That’s the pattern with human coronavirus infections.

I think the scenario… remains the most likely one, said Marc Lipsitch, an infectious diseases epidemiologist at Harvard’s T.H. Chan School of Public Health. That essentially, almost everybody has some form of immunity from natural infection and/or vaccination and/or one followed by the other, and that that will persist long enough so that they don’t get really sick when they get it again. And then we transition to endemicity.

Helen Branswell

Interesting historical perspective, but with big caveats: all pandemics of the past century were caused by influenza strains, not by a coronavirus, which may significantly change the interactions with the human immune system and the timeline to becoming endemic. Scientists are still unsure how long immunity against SARS-CoV-2 lasts, either from natural infection or vaccination. The possible emergence of more aggressive mutations, able to evade available vaccines and natural immunity, would also upset any estimates of a pandemic endpoint.

Vaccinations at the American Airlines Arena in Miami
Vaccinations at the American Airlines Arena in Miami on Thursday. Though there is consensus among scientists and public health experts that the herd immunity threshold is not attainable, it may not be all bad news. Saul Martinez for The New York Times

The examples of Brazil and India, both considered to have reached some level of herd immunity during the first 2020 wave of infection only to be hit harder a second time by a combination of mutant variants and human mismanagement, caution against early optimism. From day one, we have consistently underestimated the damage caused by the virus; it’s high time to start planning for the worst, vaccinating at a high pace to establish a safe form of immunization and continuing other mitigation measures to contain the spread.

Perhaps the clearest indication that the current pandemic is markedly different from previous influenza pandemics is how current health measures, such as mask wearing and social distancing, have almost eliminated the winter flu season in both the southern and northern hemisphere, but have only managed to slow down the spread of this coronavirus. This could be related to the lack of previous immunity, but it may mean that transitioning to endemicity will take far longer than anticipated.

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