Some doctors who got Johnson and Johnson’s COVID-19 vaccine are already getting boosters of Pfizer and Moderna, even though it is not officially recommended. Those doctors are citing the more contagious Delta variant, also called B.1.617.2, which was initially identified in India, but has since taken root in the U.S., threatening to become the predominant coronavirus variant in the states in coming weeks.

Dr. Angela Rasmussen, a virologist at Georgetown University, took to Twitter last week, writing that she initially got the Johnson and Johnson, but recently followed it up with a Pfizer booster shot after seeing the data on Delta coming from the U.K.

Rasmussen tweeted, in part: “We know that 2-dose regimens of mRNA or Astrazeneca are highly protective against the delta variant but a single dose is not.”