Beyond supply issues, though, top health officials say they are increasingly worried about the United Kingdom and South African COVID-19 variants, the likelihood that more variants will emerge in the coming months, and the possibility that those variants will evade the vaccines. There is some evidence to suggest that both the Pfizer and Moderna vaccines protect against the B117 United Kingdom variant, though a recent study shows a new mutation could make the vaccines less effective. Data gathered by the Novavax and Johnson and Johnson clinical trials in South Africa suggest their vaccines are less effective against the variant spreading rapidly in the country. And South Africa recently said it was halting the rollout of the AstraZeneca vaccine because evidence from clinical trials suggested the vaccine did not work well against the variant.

Together, the recent data has alarmed health officials in the Biden administration who are now raising questions about what more can be done to not only shorten the herd immunity timeline—not just to return Americans to some sort of normalcy but also to ensure the country does not experience another surge in COVID-19 hospitalizations and deaths. Officials have spent the last several days discussing ways to ramp up genome sequencing to track variants and how to push out the message that Americans need to more closely follow public health guidelines to reduce transmission as B117 variant cases begin to increase.

Dr. Anthony Fauci, who has previously calculated that about 75 percent of the U.S. population would need to get vaccinated to reach herd immunity, said in an interview that he is still “cautiously optimistic” that the country can achieve that goal by the beginning of the fall.