The approval allows the vaccine to be used only on adults aged 18 and above, at least six months after they have received their second dose, the ministry said in a statement.

Authorities earlier said booster shots would not be compulsory but were highly recommended for those in vulnerable and high-risk groups.

The mixing of different vaccines will also be allowed for booster doses. In addition to the Pfizer-BioNTech shot, Malaysia uses vaccines made by Britain's AstraZeneca and Chinese firms Sinovac and CanSino Biologics in its national inoculation campaign.

About 64 percent of Malaysia's 32 million population are fully vaccinated, including 89 percent of adults.

Meanwhile, Malaysia has struck a deal with US drugmaker Merck & Co. to buy 150,000 courses of its experimental antiviral pill, the health ministry said on Thursday, joining other Asian countries in a rush to secure supplies.

Molnupiravir, which would be the first oral antiviral medication for COVID-19 if it gets regulatory approval, could halve the chances of dying or being hospitalized for those most at risk of contracting severe COVID-19, clinical data has shown.

The data sparked large demand for the drug in Asia, with South Korea, Singapore and Australia announcing similar deals to buy the Merck pill this week. Taiwan and Thailand are also in talks to buy the antiviral pill.

Malaysia's Health Minister Khairy Jamaluddin said the government signed a letter of undertaking for the purchase on Thursday.

"This decision was made as we prepare to transition into an endemic phase, where we can co-exist with the virus by adding new innovative treatments as 'weapons' to fight COVID-19, apart from vaccinations and other public health measures," Khairy said in a statement.

Malaysia has recorded nearly 2.3 million COVID-19 infections, the third-highest in Southeast Asia, but has gradually lifted movement restrictions in recent weeks as the number of new cases declined amid a ramped-up vaccination program.