Residents gather to get vaccinated with the Sinovac vaccine against the COVID-19 coronavirus during a mass vaccination drive, in Semarang on Tuesday, as infections soar in Indonesia. Photo: AFP

Chinese experts called for strict protective measures despite vaccination after 350 medical workers in Indonesia who received China's Sinovac were reportedly tested positive for COVID-19 amid an epidemic spike due to the Delta mutation.

The company also warned that vaccines cannot provide 100 percent protection but could reduce severity and death.   

About 308 medical workers in Kudus, where almost all local health workers in Kudus - around 6,000 people - have received the first and second doses of a vaccine, Badai Ismoyo, head of the health office in Kudus, Central Java, a district recently hit hard by the Delta variant, was quoted as saying by local media on Sunday. 

As early as last Thursday, a Reuters report said that more than 350 doctors and medical workers in Indonesia have caught COVID-19 in Indonesia despite being vaccinated with Sinovac and dozens of them have been hospitalized, highlighting concerns over the efficacy of some vaccines against more infectious variants.

Reuters on Saturday quoted Kenneth Mak, Singapore's director of medical services, as saying that evidence from other countries showed people who had taken the Sinovac vaccine were still getting infected. "There is a significant risk of vaccine breakthrough," Mak said, referring to the report on infections of vaccinated Indonesian healthcare workers.

In response to the issue, Sinovac told the Global Times on Tuesday that vaccines could not provide 100 percent protection, but the vaccines can reduce the symptoms of infection and effectively prevent death.   

Sinovac's vaccine has effectively reduced the risk of COVID-19 symptoms in medical workers by 94 percent, according to a study by the Indonesian Health Ministry, the Xinhua News Agency reported on May 12. 

"Of this number [6,000], only 308 health workers were exposed or around 5.1 percent of the total number of health workers. Most of them have recovered and have started working again," Badai said.

Healthcare workers were among the first in Indonesia to be vaccinated when inoculations began in January and almost all have received the Sinovac vaccine.

The number of Indonesian healthcare workers dying from COVID-19 has dropped sharply from 158 in January to 13 in May, data by the independent health-linked data group LaporCOVID-19 showed.

Wei Sheng, a professor at School of Public Health of Tongji Medical College of Huazhong University of Sciences and Technology, said in an interview with China's state broadcaster CCTV on Monday that according to the results of the latest trials, our widely administered vaccines including Sinovac vaccine are still effective against the Delta variant.

It can be seen from the epidemic prevention and control experience in Guangzhou, South China's Guangdong Province that the risk of a severe disease is greatly reduced compared with those who have not been vaccinated, which in turn suggested that the vaccines are protective, Wei said.

Indonesia is one of the areas suffering the most serious COVID-19 epidemic in Asia and the country is undergoing an epidemic spike due to the spread of the severe Delta mutation that was first discovered in India. 

 The COVID-19 cases in Indonesia rose by 13,668 within one day to 2,018,113, with the death toll adding by 335 to 55,291, the Xinhua News Agency reported Tuesday citing Indonesian Health Ministry said. 

Patients at hospitals nearly crumble the country's medical system, largely increasing medical workers' infection risk, experts said. 

Many are now experiencing pandemic fatigue and taking a less vigilant approach to health protocols after being vaccinated, said Lenny Ekawati, from the Indonesian independent health-linked data group LaporCOVID-19, Reuters reported.

Chinese experts had kept calling for strict protective measures and public health management measures despite vaccination as they are the best method to prevent the transmission of the virus. 

It is also possible that some medical workers had been affected before getting vaccinated but did not show symptoms until the inactivated virus in the vaccine attacked their immune system again, experts explained.