The Food and Drug Administration authorized a second booster dose of the Pfizer / BioNTech and Moderna COVID-19 vaccines for people over 50 or who are immunocompromised, the agency announced Tuesday.

The decision comes as the United States heads into yet another wave of infections, this time driven by an even more transmissible version of the omicron variant of the virus. The FDA said in a statement that an additional dose could bolster any waning protection from the vaccines in these groups.

Once again, the FDA is leaning on data from other countries as the foundation for its decision-making around COVID-19 vaccines — in this case, data from Israel, which found a lower rate of serious illness and death in older adults who had two boosters compared with just one. But research showed while a fourth shot boosted antibody levels for younger people, it didn’t offer much protection against infection with the virus.

The US is already struggling to get first booster doses in arms: over half of fully vaccinated people haven’t received one, and a third of people over 65 have not been boosted.