The Biden administration is expected to call for a third Covid-19 shot for Americans who were fully vaccinated with the two-shot regimen, citing the threat from the highly contagious Delta variant and heightened concerns over data showing initial immunity wanes over time.

The announcement on Wednesday is likely to cover the more than 155 million people in the U.S. who have been fully vaccinated with messenger RNA vaccines from Pfizer Inc. and partner BioNTech SE or from Moderna Inc., according to people familiar with the planning. The booster shot would be administered about eight months after the second dose of the vaccine.

Regulators already cleared booster shots for people with compromised immune systems. Under the expanded plan, people 65 and older and individuals in chronic-care facilities are expected to get boosters first, followed by health workers and others in the population, the people said. The third dose will be of the same vaccine from the two-dose regimen.

The Food and Drug Administration has been weighing whether adults who received Johnson & Johnson ’s single-dose shot will also need another Covid-19 vaccine shot, another person familiar with the situation said. Data about J&J’s shot isn’t complete but it is likely this population will also need booster shots, one of the people said.