SINGAPORE : People who received two doses of China's Sinovac vaccine were more likely to develop severe disease from Covid-19 than those who got two shots of the mRNA-based Pfizer-BioNTech or Moderna vaccines, the National Centre for Infectious Diseases (NCID) has found.

"This supports the need for three doses of Sinovac-CoronaVac vaccine as a primary series," it said in a statement on Wednesday (Dec 15).

Researchers found that the Sinovac vaccine was 60 per cent effective against severe disease, compared with 90 per cent for the Pfizer vaccine and 97 per cent for Moderna's.

In other words, people who took the Sinovac vaccine were more likely to require supplemental oxygen and intensive care, and also more likely to die of the coronavirus.

The study of 1.25 million people aged 40 and above was carried out between Oct 1 and Nov 21.

Of this group, 62,900 people eventually became infected with Covid-19 and 1,710 fell severely ill.

The study focused on the Delta variant - which is the dominant strain in Singapore - and accounted for differences in age, gender, race, housing type and the daily differences in infection rate.

NCID suggested that people who have taken two doses of the Sinovac vaccine should get either the Pfizer or Moderna vaccine as a third dose in their primary vaccination series. Otherwise, they should get a third dose of Sinovac.

It noted that some international data backs up its findings.

Preliminary data from Chile shows that people who took two doses of the Sinovac shot and then received the Pfizer vaccine as a third dose showed a 95 per cent reduction in their risk of infection.

In contrast, the risk reduction in persons who received Sinovac as their third dose was 71 per cent.

The increase in antibodies for the first group was also 27 times higher than in the second group.

"We will continue to monitor the vaccine effectiveness of the mRNA and non-mRNA vaccines, including how they are enhanced by a third dose or a booster dose," NCID said. "In the meantime, all persons who are eligible for their boosters should get boosted when it is offered to them."

  - ANN