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Herd Immunity is Almost Here. What Next?

In the near future, likely before Christmas, 90 percent or more of the U.S. population will have considerable — but not perfect — immunity to the novel coronavirus, SARS-CoV-2.

Close to 70 percent of Americans have had at least one vaccine dose, while a very substantial if not precisely known percentage of the unvaccinated have recovered from infection. The so-called herd immunity target will be achieved. But what will this actually mean?

For a virus such as measles, our immunity is so good that it almost never infects a protected individual. Herd immunity means that so many individuals have achieved protection that their collective immunity prevents the virus from circulating in that population. For SARS-CoV-2, herd immunity is unfortunately more illusory. Even in the best of circumstances, covid-19 will continue to cause sickness and occasionally death.

Where does that leave us? While no one can predict with certainty what will happen, there are several reasonable scenarios in the near and medium terms. Over the long term, it is harder to say.

In the best short-term scenario, variants will continue to emerge but won’t have significant impacts. Delta now causes 99 percent of U.S. cases; the once-feared alpha variant is nowhere to be seen, and neither are variants that once seemed threatening after they emerged in New York, California, South Africa and elsewhere. Delta, in this scenario, will continue to outcompete any variant that emerges, and severe disease will be limited to the steadily decreasing few who still lack any immune protection. Breakthrough infections will still occur, but the immune systems of those vaccinated or previously infected will protect them against severe disease and death.

In this scenario, based on current death rates in breakthrough infections, covid-19 might very well become less threatening than seasonal influenza. This could happen sooner rather than later.

A less favorable short-term scenario would see the emergence of as-yet-unknown variants better at evading our immune systems than those we have encountered so far. In vulnerable populations lacking any immune protection, delta has elbowed such variants aside. But once the vast majority of people have acquired significant immunity, the competitive advantage could shift to variants better geared to evade immune protection — at least for a while.

That is largely what happened in 1918. As the virus adapted itself to humans, it caused three waves. The second wave was far more transmissible and lethal than the first, and it was followed by a third wave in 1919 that had no problem infecting people previously sickened. This third wave was less lethal, probably because of a combination of viral mutations and moderate protection provided by the immune system. By 1921, the once-lethal virus was transformed into seasonal influenza, killing relatively few over the following decades and having no impact on daily life.

Something like this could occur with covid-19. If so, at least one more wave is coming. It’s likely our immune systems would still protect us against severe illness and death, but, as we do with influenza, we would need to update vaccines to keep pace with mutations. Eventually, life would return to pre-pandemic normal.