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Booster shots recommended after six months: experts

By Cam Le, Chi Le   November 3, 2021 | 04:47 pm PT
Booster shots recommended after six months: experts
A boy receives a Covid-19 vaccine shot in HCMC's District 1, October 27, 2021. Photo by VnExpress/Quynh Tran
Efficacy of the second Covid-19 vaccine shot would gradually drop after six months, requiring a third to prevent infection, experts warned.

Do Van Dung, head of the public health department of the Ho Chi Minh City University of Medicine and Pharmacy, on Tuesday said he supported Covid-19 vaccine booster shots, especially for health workers who face a higher risk of infection.

The adaptive immune system could "remember" pathogens that had attacked the body and retaliate against them later using antibodies, said Dung. But the amount of antibodies that fight off the coronavirus would drop by half after around 108 days, or 15 weeks, in vaccinated individuals, according to an Australian research.

Combined with other factors, if the efficacy of a Covid-19 vaccine is at around 90 percent in the beginning, it could drop to 70 percent six months later, Dung said.

Data by Pfizer back in July on the effectiveness of vaccines on 44,000 people revealed the general efficacy of the second shot would wane from 91 to 84 percent after six months. Effectiveness peaks at around 96.2 percent from one week to two months after the shot, and would gradually drop by 6 percent every two months, according to Pfizer. However, its effectiveness in preventing severe Covid-19 remains stable at 97 percent, it added.

When there are fewer antibodies and one is exposed to the coronavirus, re-infection could occur. While severe cases would be unlikely, the virus would continue to replicate inside the body and may infect others, sparking new outbreaks. Therefore, booster shots are important to prevent infections, Dung said.

Calvin Q Trinh, a doctor at 1A Hospital in HCMC, also supports booster shots as antibody levels would surely drop over time. He cited the U.S. giving people a third shot of the Moderna or Pfizer vaccines six months after getting the second.

"Vietnam is currently using multiple types of Covid-19 vaccines, and some of them don't have very high protection effectiveness (50-60 percent according to published reports). So based on different circumstances, booster shots could be given earlier, maybe four to five months after the second shot," he said.

Booster shots should not only be for those at the frontline of the Covid-19 fight, but also immunocompromised individuals like those with cancer or HIV, those with underlying conditions and those who work in tourism and related services, among others. Eventually, the entire population should get booster shots, he said.

Authorities would need to secure enough vaccine doses for booster shots. However, one should avoid getting three shots each from a different type of vaccine as there has been inadequate research to see what would happen in such scenarios, he added.

Dung said there should be more research to consider what kind of vaccine would be used as booster shots, who should receive them and when. He anticipated that booster shots would not be required every single year, but around once every three to five years, similar to other types of vaccines, as the coronavirus is unlikely to mutate fast enough to evade the immune system.

On Oct. 30, the HCMC Department of Health proposed to the municipal People’s Committee and the health ministry to approve booster shots for those at risk of getting Covid-19, including frontline Covid fighters.

Vietnam has vaccinated 58 million people with at least one Covid-19 vaccine shot. A total 25.1 million people have been fully vaccinated.

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