The administration of US President Joe Biden has revealed plans to provide coronavirus vaccine booster shots to those who have received two doses.
The government launched a vaccination rollout in December last year. As of August 17, 168.9 million people, accounting for 50.9 percent of the US population, are fully vaccinated.
The spread of the highly contagious Delta variant, however, has caused the number of new cases to soar above what they were during the summer peak of the outbreak in the US last year.
The Department of Health and Human Services on Wednesday released a statement saying vaccines continue to be effective in reducing the risk of severe disease.
But it says available data "make very clear" that protection against infection begins to decrease over time following the initial doses of vaccination.
It says a booster shot will be needed to maximize and prolong vaccine-induced protection.
It says booster shots will be offered beginning the week of September 20 and starting eight months after an individual's second dose.
Eligible for a third shot are people who were inoculated with the vaccine jointly developed by Pfizer and its German partner BioNTech, or the one created by Moderna.
Health care providers and seniors are to begin receiving the shots.
Authorities also say they require several more weeks to receive data on Johnson & Johnson vaccine recipients to plan a booster shot for them as well.
The statement, co-authored by the director of the Centers for Disease Control and Prevention, Rochelle Walensky, says the "top priority" is to protect the American people.
Meanwhile, the chief scientist of the World Health Organization, Soumya Swaminathan, expressed a different view to reporters on Wednesday.
She said data does not indicate that boosters are needed for everyone.
The WHO has been calling on nations not to administer vaccine boosters until at least the end of September in a bid to promote inoculations in developing countries and elsewhere.