How often do we have to get Covid to stop getting Covid?

Relegating SARS-CoV-2 to the status of an endemic virus may require lots and lots of us getting the disease

In October 2020, a few weeks before the experimental trial results for the BioNTech-Pfizer, Moderna and Oxford/AstraZeneca Covid-19 vaccines were released, German virologist Christian Drosten cautioned that the shots would be of limited effectiveness in preventing the spread of the disease.

“We are dealing here with an infection of the mucous membrane, i.e., in the nose and the throat and then later the lungs,” he said on Episode 62 of Das Coronavirus-Update, the podcast launched by broadcaster NDR in March 2020 that helped make Drosten a household name in Germany. “The mucous membranes already have their own special local immune system. With the current vaccines, which are more likely to be injected into the muscle, you don’t reach this local immune system so well.” As a result, the vaccines “probably protect more against the severe course [of the disease] than against infection.”

Which is of course exactly how things played out. The vaccines have been spectacularly effective at preventing severe disease and death, much less so at preventing transmission.

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Drosten has been described in the US media as “Germany’s Fauci,” but the moniker seems a little unfair. National Institute of Allergy and Infectious Diseases Director Anthony Fauci was a top researcher in his day, but has been running a sizable government agency (fiscal year 2021 budget, $6 billion) for the past 38 years. His appearances in the media during the pandemic have mostly involved reciting the public-health consensus of the moment, which was useful when the US president was a font of Covid-19 misinformation but not always especially enlightening. Drosten, director of the Institute of Virology at Berlin’s Charité medical school, is perhaps the world’s leading coronavirus expert, responsible for identifying the original severe acute respiratory syndrome virus in 2003 and devising the first diagnostic test for the Covid-causing SARS-CoV-2 virus in January 2020. His public commentary, delivered chiefly in podcast form after he grew frustrated dealing with some in the German media, has tended toward the cutting-edge and forward-looking — as indicated by what he was saying about vaccines in October 2020.

I recount all this as context for what Drosten said last month in the 113th and final (for now) regular Coronavirus-Update episode. He was talking again about the mucous membrane — which in German is conveyed as the more graphic “Schleimhaut,” literally “slime skin” — and its role in keeping infectious diseases in check (translation and editing-for-brevity by me):

With influenza it is simply the case that everyone gets infected x times over the course of their lives. These infections occur in the mucous membrane, in the throat. Our mucous membranes have a local immune system of their own, if you want to call it that. And everyone in the population, except for the children, of course, has so many infections behind them that there in the mucous membrane, immunity exists. That is why the adults in the population, and that is the vast majority of the population, are not so infectious.

It is this mucosal immunity that keeps influenza from spreading most of the year, with the disease’s effective reproduction number (the average number of people who will get it from each one who has it) surpassing 1 for only a few months in winter, when indoor crowding, drier air and other factors seem to lead to increased transmission.

Contrast that with Covid: Current vaccines generate some mucosal immunity but it fades quickly, and while there are nasal-spray vaccines in development that target the mucous membrane, they’re not ready yet, and this approach has been of only mixed effectiveness against influenza. Meanwhile, less than half the German population has been infected with Covid, Drosten said (in the US, according to a Centers for Disease Control and Prevention report this week, it’s 57.7 per cent), so there probably isn’t nearly enough mucosal immunity out there to prevent the reproduction number from rising back to 2 or 3 in the fall, bringing another huge wave of infections and concomitant stresses on hospitals and disruption of daily life.

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One thing that could preclude this, Drosten speculated, might be a lot of young people (the “Party Generation,” he called it, in English) getting infected a second or third time over the summer, but he expressed doubt that this would be enough to make a difference in 2022. How many times are people going to need to be infected to confer effective immunity against transmission, asked NDR science reporter and podcast moderator Korinna Hennig. Drosten’s reply:

My idea is that this is in the range of a number that you can count on one hand. But no one can say for sure at the moment.

The role of infection-conferred immunity in slowing the spread of Covid has been a sensitive topic. Early in the pandemic, it was emphasized by a few scientists (and a lot of non-scientists) who wildly underestimated the deadliness of the disease. Later, after vaccines became available, anti-vaxxers argued that natural immunity was superior to the sort conferred by shots. The latter claim isn’t entirely wrong — as might be expected from Drosten’s explanations, there’s some evidence that prior infection provides better protection against subsequent infection than current vaccines do (although there’s counter-evidence, too). But it’s far less dangerous to get a shot than to get Covid for the first time, and once you’ve been infected, vaccines add significant protection against reinfection.

Relegating SARS-CoV-2 to the status of an endemic virus like the flu or the several coronaviruses that cause common colds, though, may require lots and lots of us getting the disease, again and again. As Drosten put it in an interview in January:

Over the long run, we will not be able to vaccinate the population every few months. That won’t do. At some point, the virus itself will have to keep updating people’s immunity.

Given the source, it’s hard to argue with this. As a guide to policy and behavior, though, it’s not so clear. Going to great lengths to avoid infection makes less sense for most of us than it used to, and trying to maintain a zero-Covid policy as a nation, as China is doing, seems to make no sense at all. But should we be going out and trying to get infected?

A few weeks ago, after writing about my belief that wearing masks on buses and subways ought be encouraged even after the mandates go away, I heard from readers who argued that this was unwise because getting exposed to germs on the bus or subway builds immunity. But if that’s the case, the great majority of Americans who aren’t fortunate enough to commute by public transportation must be putting themselves in grave danger. By that logic, don’t they need to hang out unmasked in enclosed spaces crowded with strangers for a couple of hours every week?

Put it that way, and the argument for not ever bothering to take precautions falls apart. Drosten certainly doesn’t subscribe to it, instead expressing hope in the final Coronavirus-Update that the “Asian courtesy” of wearing a mask when ill and in certain other situations catches on in Germany. Wondering what the right balance might be, I asked my primary-care physician (and college classmate) Bertie Bregman, who has been treating Covid patients in New York since the early days of the pandemic and contracted the disease early on, too. He had an interesting response: “In a nutshell, what I believe is that we have to stop being so neurotic about Covid and be more neurotic about everything else.”

That is, with Covid now manifesting itself among the vaccinated and those with prior infections (i.e., almost everybody) mainly as a standard-issue upper-respiratory infection, we should probably just treat it as a standard-issue infection, albeit with a heightened awareness that the spread of such diseases can and should be slowed by simple measures like staying home and wearing a mask when sick. The one “wild card,” Bregman allowed, is Long Covid, the risk that even mild cases now could lead to complications later. But taking reasonable care around cold-and-flu-like symptoms is likely to be more effective in curbing Covid, he thinks, than the current widely followed approach of taking rapid Covid tests and going about life as normal if they come back negative — “then half the time a week later they turn out to be positive.”

What about building immunity? Exposing children early on to germs and allergens pays dividends later, Bregman said, but for adults “the costs of getting sick outweigh the benefits.” So yes, most of us will probably get Covid, repeatedly. No need to rush it, though.

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