The Delta variant is now driving outbreaks among unvaccinated populations in countries like Malaysia, Portugal, Indonesia and Australia. Delta is also now the dominant variant in the United States, the C.D.C. reported this week.
Until recently, infections in the United States had plateaued at their lowest levels since early in the pandemic. Hospitalizations and deaths related to the virus have continued to decline, but new infections may be rising.
It’s not yet clear to what extent the variant is responsible; a slowing vaccination drive and swift reopenings also are playing roles.
Citing data from Israel, Pfizer and BioNTech suggested their vaccine’s efficacy “in preventing both infection and symptomatic disease has declined six months post-vaccination.” Noting the rise of Delta and other variants, the companies said that “a third dose may be needed within 6 to 12 months after full vaccination.”
Health officials in Israel have estimated that full vaccination with the Pfizer-BioNTech offers only 64 percent efficacy against the Delta variant. (Efficacy against the original virus is greater than 90 percent.)
But Israel’s estimates have been contradicted by a number of other studies finding that the vaccine is highly effective at preventing infection — against all variants. One recent study showed, for example, that the mRNA vaccines like Pfizer’s trigger a persistent immune reaction in the body that may protect against the coronavirus for years.
“Pfizer looks opportunistic by hanging an announcement on the back of very early and undigested data from Israel,” said John Moore, a virologist at Weill Cornell Medicine in New York. “When the time is right for using boosters here, the decision isn’t theirs to make.”