A Covid-19 vaccine specifically targeting the Omicron variant would be "prudent" even if it ultimately may not be needed, the top US infectious disease official said on Tuesday as Pfizer announced trials for such a vaccine.

"It makes sense to think in terms of at least having ready an Omicron specific boost," Dr Anthony Fauci, President Joe Biden's top medical adviser and a member of the White House Covid-19 Response Team, told MSNBC.