Philippine President Rodrigo Duterte may be injected with Russia's coronavirus vaccine as early as May 2021, according to the government's official newswire, which quoted the presidential spokesman on Thursday.

Earlier this week, Duterte volunteered to participate in trials for the Russian vaccine, even as scientists and health experts questioned the safety and effectiveness of the drug.

Russia on Tuesday announced it had registered the world's first vaccine for the fast-spreading coronavirus disease, or Covid-19, after less than two months of clinical trials.

Phase three trials are considered by medical experts to be critical for any vaccine development and are needed to root out any potential side effects.

Stephen Morrison of the Center for Strategic and International Studies told CNBC's "Squawk Box Asia" Wednesday that it's no surprise Russian President Vladimir Putin offered Russia's vaccine to Duterte.