Sources told the Washington Post the FDA could make the announcement at the same time it authorizes booster doses for some Moderna recipients, and all Johnson & Johnson (J&J) recipients.

The move is based on early data presented to the FDA late last week from the National Institutes of Health, as well as evidence from the European Union and United Kingdom, where mixed dosing has been used successfully.

The announcement will likely offer guidance to the 15 million Americans inoculated with the Johnson & Johnson (J&J) vaccine, which has proved less effective than either the Pfizer or Moderna vaccines. Though J&J recipients could choose to get a second dose of that vaccine, they could also choose to mix their initial adenovirus-based vaccine with an mRNA shot. This strategy, called heterologous prime-boosts, has been shown to be well-tolerated.

Currently, the FDA has unanimously approved a J&J booster, but later this week the Centers for Disease Control and Prevention (CDC) advisory group, ACIP (Advisory Committee on Immunization Practices), will meet to discuss booster shots for Moderna and J&J recipients.

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