Omicron's best-case scenario

If Omicron continues to show signs of being milder than Delta, that’s good news, of course. But if it also turns out to spread more quickly than Delta, that could be great news. When two variants are circulating, the one that infects more people more quickly will tend to dominate, said Samuel Scarpino, of the Rockefeller Foundation’s Pandemic Prevention Institute. That variant could win out either because it replicates more quickly in its human hosts and spreads more efficiently between them—that is, it’s more transmissible—or because it more deftly evades the immunity we already have.

Immune escape sounds especially scary; after such a hard-fought battle to stay healthy until the vaccines arrived, or a hard-fought battle against the virus itself, no one wants to be told they’re susceptible again. But a reinfecting variant that doesn’t come with the risk of chronic symptoms or ventilators might not be such a bad thing, Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center, told me. “If it can get around the vaccines, but in the end really causes less severe disease, that’s probably a step in the right direction.”

One possible downside to a super-contagious (or immune-evading), super-mild Omicron would be that those who catch it won’t end up with much protection afterward, Scarpino said. As my colleague Katherine J. Wu has reported, mild COVID cases may not lead the immune system to produce as many antibodies as do more serious illnesses. But Ali Ellebedy, an immunologist at Washington University in St. Louis, told me that mild infection doesn’t necessarily preclude a robust immune response.