Scientists at the US Food and Drug Administration (FDA) are strongly against administering only single doses of the Moderna and Pfizer vaccines, as their efficacy has only been proven if people get two shots, The Wall Street Journal reports.

"It is essential that these vaccines be used as authorized by FDA in order to prevent Covid-19 and related hospitalizations and death," Peter Marks, director of the FDA center that oversees vaccines, told The Wall Street Journal on Saturday. In February, some scientists and lawmakers suggested using a one-dose regimen for all coronavirus vaccines approved in the US, after certain preliminary studies showed that a single shot could be enough for some people.

"You would be flying blind to just use one dose," a senior scientist and adviser to US President Joe Biden told The Wall Street Journal, adding "If you're going to do something else other than follow the studies shown to the FDA, show me that this one-shot effect is durable."

At the end of February, the FDA announced that it had given emergency use authorization to Johnson & Johnson's single-shot vaccine. The coronavirus vaccine was the third candidate to be given FDA approval, following authorization for two-shot vaccines produced by Pfizer and Moderna.

Biden said after the approval of Johnson & Johnson's vaccine that things were still likely to get worse in terms of COVID-19 in the US as new coronavirus variants emerge.