A boost for boosters The evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern poses a potential obstacle to achieving vaccine-induced immunity. Pegu et al . examined how viral variants, including the B.1.351 (Beta) and B.1.617.2 (Delta) variant, affected the immune response in a small number of individuals who received the Moderna mRNA-1273 vaccine. By analyzing sera obtained 6 months after the second shot in the primary vaccine series, the researchers found that neutralizing antibody titers persisted against all variants tested. However, neutralizing antibodies against the B1.351 variant had dropped considerably by 6 months, and some individuals had weak, and in some cases no, neutralizing activity. These data may help to guide public health policies regarding additional booster vaccinations. —PNK