Dr. Kavita Patel, a primary care physician in Washington, D.C., said she believes the people who got the J&J vaccine have been left “high and dry,” injected with a vaccine that may not be sufficiently efficacious against delta but lacking a government-sanctioned way to get an extra shot.

“We’ve been talking a lot about third doses, but we’ve forgotten there are 13 million people who don’t have a third-dose option because they got J&J,” she said, adding she has not formally advised her own patients to get boosters.

In a statement Tuesday night, Johnson & Johnson said its single-dose vaccine provides two mechanisms of protection — antibody and T-cell immune responses — that “persisted through eight months after immunization.”

What’s more, a real world study of health care workers in South Africa previewed Friday suggests that the Johnson & Johnson vaccine is 71 percent effective against the delta variant at preventing hospitalization and over 90 effective at preventing death.