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Why U.S. COVID cases are plateauing even though millions are getting vaccinated

March 23, 2021, 8:34 PM UTC
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Viruses don’t really have a nationality. Sure, certain strains of a pathogen may initially stem from one geographic region. The epicenter of an outbreak is by definition the ground from which a pandemic begins to spread. But ultimately it’s within a virus’s nature to mutate in order to survive, wherever that mutation may first occur. And emerging new coronavirus strains are likely a major reason that a steady decline in new COVID cases has plateaued in the U.S. despite an aggressive vaccination drive.

As of March 23, more than 164 million COVID vaccine doses had been delivered and more than 128 million shots had gone into arms. More than 45.5 million Americans have been fully vaccinated (i.e., received the single shot Johnson & Johnson vaccine or the dual-shot Pfizer or Moderna vaccines) and more than 25% of the population has received at least a single dose. More than 2 million doses are being administered per day, on average. That puts the U.S. well ahead of European nations in its immunization campaign despite a rocky initial rollout and lingering inequities in vaccine access.

That’s helped lead to a significant decline in the seven-day moving average of new reported COVID cases. On March 22, there were 55,902 new infections, according to the Centers for Disease Control (CDC). That’s still a much better situation than what was happening back in early January, when there were regularly close to 300,000 (if not more) cases per day.

But it’s important to look at the seven-day moving average of cases, seeing as it takes time for a coronavirus infection to set in and be detected, as well as regional reporting differences. After January 2021 peaks of about 250,000 average cases per day, the U.S. began seeing a steady decline, falling to about 65,000 average cases per day. And then, the rate of decline stalled in mid-March.

CDC COVID 7 day average
Centers for Disease Control

But why? Medical experts such as Brown University School of Public Health dean Ashish Jha point to variants as a major potential driver.

Indeed, the latest available CDC data finds that the B.1.1.7 coronavirus variant (otherwise known as the U.K. variant) is presently the most common coronavirus mutation in the U.S. since it’s more transmissible than other strains. As of March 21, there were nearly 6,400 reported cases of people infected with that variant. There have been another 194 cases of the B.1.351 variant that have been detected (the South Africa strain), as well as 54 cases of the P.1 variant (the Brazil strain).

In New York City, which is reemerging as an epicenter of coronavirus spread and has its own local strain of the virus, these newer versions of the microbe accounted for 51% of all current cases as of March 10, according to city health officials.

That’s cause for concern and continued vigilance in wearing masks and socially distancing as we see how effective the Moderna, Pfizer, and Johnson & Johnson vaccines are in protecting against evolving coronavirus strains. The tentative good news is there’s reason to believe they provide at least some protection.

“So far, studies suggest that antibodies generated through vaccination with currently authorized vaccines recognize these variants. This is being closely investigated and more studies are underway,” writes the CDC, adding that basic public safety measures are still necessary.

Furthermore, the Moderna and Pfizer vaccines in particular may be more convenient to transform into booster shots that specifically target variants because of their underlying technologies. Instead of having to create an entirely new, conventional vaccine from scratch, sequencing the genetic code of a mutated virus would allow the firms to leverage their mRNA-based vaccine platforms to more rapidly create a booster. And the search is on around the globe to create a universal coronavirus vaccine.

But with warmer weather quickly approaching, it’s clearly no time for Americans to throw public health caution to the wind.