Thailand and Indonesia have begun recommending U.S.- and European-made coronavirus vaccines to their populations after seeing some deaths among those inoculated with Chinese vaccines amid the spread of the highly transmissible Delta variant.

The abrupt change of tack in the two Southeast Asian countries reflects concerns about the efficacy of Chinese-made vaccines, such as the one produced by China's Sinovac Biotech Ltd., against the variant first identified in India and spreading quickly across the region and beyond.

Southeast Asia has relied heavily on Chinese-made vaccines. But Thailand said on July 12 that it would give the vaccine produced by Britain's AstraZeneca Plc as a second dose to people who received a first dose of the Sinovac vaccine.

A woman is administered with the AstraZeneca coronavirus vaccine in Bangkok on July 13, 2021. (Getty/Kyodo)

The Thai government also unveiled plans to give those who have already completed a two-dose regimen a booster shot with vaccines from drugmakers such as AstraZeneca and U.S. pharmaceutical company Pfizer Inc.

According to the government, 618 of the roughly 677,000 medical workers who received two doses of Sinovac on a preferential basis between April and July have become infected, leading to the death of one nurse.

The government said the moves are intended to enhance people's immunity to the Delta variant and did not cite efficacy issues.

A doctor at a Bangkok hospital indicated that people are still advised to get vaccinated even with Sinovac, saying inoculation should be expedited. But a 37-year-old man in the capital who is fully immunized said he became worried after hearing about the nurse's death.

Another male, aged 41 and unvaccinated, said he was waiting for the vaccine made by U.S. pharmaceutical company Moderna Inc., citing his lack of confidence in Sinovac.

In Indonesia, which recently saw new daily infections surge at the fastest pace in the world, over 80 percent of the roughly 152 million doses of vaccine secured by the government as of July 22 have been Sinovac.

On July 16, the government began giving medical workers who had already received two shots of vaccine an additional shot using the Moderna vaccine.

Health Minister Budi Gunadi Sadikin has said booster shots are intended to give frontline medical workers "maximum protection against new variants of the virus." The minister did not refer to efficacy issues.

But since January, 20 doctors have died of COVID-19, the disease caused by the virus, despite receiving at least one vaccine shot, and some of them had been inoculated with Sinovac, according to a doctors' association in the country.

In Singapore, where the government provides Pfizer and Moderna vaccines as part of its free vaccination program, Sinovac has also been made available through private clinics to individuals who wish to have it.

But the government has so far excluded people vaccinated with Sinovac from its official vaccination tally.

"We don't really have a medical or scientific basis or have the data now to establish how effective Sinovac is in terms of infection and severe illnesses on Delta," Health Minister Ong Ye Kung explained at a media briefing earlier this month.

The World Health Organization has given emergency-use approval to Sinovac and a vaccine developed by Chinese pharmaceutical giant Sinopharm.

The WHO cited Sinovac's 51 percent effectiveness in preventing symptomatic COVID-19, which is lower than the more than 90 percent efficacy rates of the Pfizer and Moderna vaccines and the around 70 percent for AstraZeneca's.

China has provided over 100 million doses of vaccines to countries in Southeast Asia. Chinese vaccines account for most of the doses secured by Cambodia, Indonesia and Laos.