"We still believe the best approach is to use the same vaccine for the two primary doses," said Dr. Alejandro Cravioto, chairman of the panel, at a briefing on Thursday, Reuters reported.
Vaccine combinations, already used by some governments, could help low- and middle-income countries manage stockpiles and deal with vaccine shortages as the new Omicron variant spreads.
European Union regulators endorsed mixing two different COVID-19 shots for initial vaccine schedules and boosters on Tuesday.
Dr. Cravioto said if countries mix vaccines, the best approach is to use a second dose of a messenger RNA or vector-based vaccine if the first dose was an inactivated vaccine. Messenger RNA vaccines are best followed by vector-based ones, he also said.
A study published by the University of Oxford earlier this week showed that mixing other vaccines with those from AstraZeneca or Pfizer offered at least as effective protection against COVID-19 as giving two doses of the same shots.
The WHO panel also recommends that people who are immunocompromised or received an inactivated vaccine should receive a booster dose of a COVID-19 shot.
Many countries have been rolling out booster shots, targeting the elderly and people with underlying health issues, but worries about the new Omicron variant have prompted some to expand their use to larger portions of their populations.
With vaccination rates worryingly low in much of the developing world, the WHO has said in recent months that administering primary doses - rather than boosters - should be a priority.
Dr. Cravioto said emerging data showed that vaccines' efficacy against COVID-19 wanes, with a significant decline seen in older people in particular.
COVID-19 vaccines protect "very well" through six months after the last dose with some "minor, modest reduction" in protection, Dr. Kate O'Brien, director of the WHO's immunization department, said.
Inactivated vaccines which take the Sars-CoV-2 virus and inactivate or kill it using chemicals, heat or radiation, are made by Chinese manufacturers Sinovac Biotech, state-owned Sinopharm and India's Bharat Biotech.
A single dose of the Johnson & Johnson vaccine is still effective, but data from the company's clinical trials using two doses clearly show the benefit of having further vaccination, Dr. Cravioto said.