GENEVA — The head of the World Health Organization called on drugmakers to prioritize supplying their COVID-19 vaccines to poor countries instead of lobbying rich countries to use even more doses, just as some pharmaceuticals are seeking authorization for a third dose to be used as a booster.

At a press briefing on Monday, WHO chief Tedros Adhanom Ghebreyesus said the massive disparity in vaccines between rich and poor countries means that “we are making conscious choices right now not to protect those in need.” He said the priority now must be to vaccinate people who have received no doses.

Tedros called on Pfizer and Moderna to “go all out to supply COVAX, the Africa Vaccine Acquisition Task Team and low and middle income countries with very little coverage,” referring to the U.N.-backed initiative to distribute vaccines globally. Both Pfizer and Moderna have agreed to supply small amounts of their vaccine to COVAX, but the vast majority of their doses have been reserved by rich countries.

Last week, Pfizer said it would seek authorization for a third dose of its COVID-19 vaccine, saying a booster shot could dramatically boost immunity and perhaps help ward off worrisome variants.