Thailand is considering switching from a Chinese to Western vaccine to protect healthcare staff.
The country is considering giving AstraZeneca doses to those already vaccinated with Sinovac.
It came after 600 vaccinated staff caught COVID-19, adding to questions surrounding Chinese jabs.
Officials in Thailand appear to be losing confidence in the Chinese-made vaccine it gave its healthcare workers, recommending that they receive booster shots of the AstraZeneca jab.
The recommendation came on Monday from the nation's National Communicable Disease Committee. It was made after more than 600 medical personnel contracted COVID-19 despite having two doses of the Sinovac jab.
The decision is one of several taken recently to move away from Sinovac, and comes as the vaccine's effectiveness is being questioned more broadly.
Indonesia on Friday announced it would give a Moderna shot to health workers who were fully vaccinated with Sinovac.
It followed Bahrain and the UAE also offering a third shot to people who took Sinovac initially.
According to data from the Thai Health Ministry released on Saturday, 677,348 Thai medical personnel got two doses of Sinovac vaccine between April and July 10.
Of those, 618 were infected with the coronavirus, in spite of being fully vaccinated.
The majority of those - 597 of 618 - either had no symptoms or only a mild form of COVID-19. Nineteen contracted a moderate form of the disease, the figures said.
One nurse died, and another health worker was described as being in critical condition.
Last month, Indonesia reported that more than 350 health workers that were fully vaccinated with the Sinovac vaccine got COVID-19, and that dozens of them had been hospitalized.
Ten Indonesian doctors who died of COVID-19 in June had been fully vaccinated, the Guardian reported.
The news comes as Thailand is imposing lockdown-like measures to contain its deadliest COVID-19 outbreak to date, which is fueled by the Delta variant, Al Jazeera News reported on Monday.
Questions remain about the vaccine's efficacy, especially against the Delta variant
Whether the Chinese vaccines can protect against the Delta variant has been questioned in several news outlets, including Insider, after highly vaccinated countries relying on these vaccines saw surges in cases.
The efficacy of the vaccine is not clear, as different trials have produced different results.
Data from a Turkish trial, which was published in the peer-reviewed journal The Lancet on Thursday, found the vaccine to have 83.5% efficacy against symptomatic infection.
This trial took place when the Alpha variant, first seen in the UK, was dominant in Turkey.
Real-world data from Indonesia, which monitored about 128,000 vaccinated healthcare workers between January and March found the vaccine was a lot more protective: 94% protection against infection and 96% effective at preventing hospitalization.
This was before the Delta variant, which is more resistant to vaccines, was in the country.
A less positive trial, this time in Brazil, puts Sinovac's efficacy at about 50%, at a time when another variant, Zeta, was dominant in the country.
China is investigating the efficacy of its shots against the more contagious Delta variant, the Wall Street Journal reported on July 9.
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