‘Effectiveness against Covid gradually decreases after 6 months of second dose vaccination’ – VnExpress

6 months after the two doses of Covid-19 vaccine, the protective effect is still 70% (assuming the initial effectiveness is 90%, depending on the type of vaccine), experts believe that this time the booster shot will help limit the spread of the virus. lan.

Reply VnExpress On November 2, Assoc. Prof. Dr. Do Van Dung (Dean of the Faculty of Public Health, University of Medicine and Pharmacy in Ho Chi Minh City) supported the three-shot Covid-19 vaccine, especially for the group of health workers at high risk due to the virus. frequently in contact with infected patients.

Mr. Dung explained, antibodies, T and B cells with immune memory form a “shield” against Covid-19. In particular, antibodies play an important role in protecting against symptomatic Covid-19, and T and B cells have protective immune memories that keep the disease from getting worse or causing death.

An Australian study estimated that anti-Covid-19 antibodies will halve after 108 days (about 15 weeks), based on antibody data in vaccinated people. However, other immune cells were not reduced, even B cells with memory increased slightly over time during the first 6 months. Therefore, assuming that the vaccine effectiveness against symptomatic Covid-19 infection at baseline is 90%, then 6 months after vaccination, the effectiveness is 70%. That is, the overall protective effect is reduced by 20%, and the level of antibodies produced by the vaccine can be reduced even more.

The most recent Pfizer data (July 2021), based on vaccine efficacy assessments performed on 44,000 people in the US and other countries, showed a decrease in overall vaccine effectiveness after two injections from 91% to 84% after 6 months. The study also showed that the effectiveness of the Pfizer vaccine was strongest at 96.2% between one week and two months after the second dose, with an average reduction of 6% every two months. The vaccine effectiveness is stable at 97% for the prevention of severe Covid-19 disease.

According to an announcement by Moderna in August 2021, Moderna’s vaccine is overall effective at 93% for 6 months and 98% for severe Covid-19 disease after receiving two full doses. However, these latest data do not assess efficacy against the Delta strain.

Currently, there are no published studies on the long-term efficacy of the real-world vaccines of AstraZeneca and Sinopharm’s Covid-19 vaccines.

Thus, after about 4-6 months of vaccination, antibodies decline and if exposed to nCoV, people can get sick. Although not seriously ill, the virus still reproduces in the body, so it can spread to others, creating an “opportunity” for an outbreak. Therefore, the booster shot is very important to ensure no infection, according to Associate Professor Dung.

Pfizer vaccination for children over 12 years old at Luong The Vinh High School, District 1, Ho Chi Minh City, October 27 Photo: Quynh Tran

Master, doctor Calvin Q Trinh, 1A Hospital in Ho Chi Minh City also supports the three shots of Covid-19 vaccine because antibody levels will definitely decrease over time. For example, in the United States, the time for a booster shot is 6 months after completing the second dose of Pfizer-BioNTech or Moderna.

“Vietnam is injecting many vaccines, in which some types of protection are not high (50-60% according to published research), so depending on the actual situation, the booster dose may be given earlier, 4-5 months after the nose, “the doctor said.

The doctor said that priority should not only be given to booster doses for the frontline against the epidemic, but also for immunocompromised groups such as cancer patients, HIV-infected people, people with underlying diseases, people working in tourism, etc. service… then deploying the injection to the whole population. The management agency needs to calculate the appropriate vaccine import to meet the demand for the triple injection along with one of the two types of people who have been vaccinated. Avoid the case of three completely different injections because the situation has not been studied.

Besides, PGS Dung also said that it is necessary to have a thorough evaluation study before mass vaccination, for example which vaccine, which group of people need the third injection, and when. Experts predict that there is no need for an annual Covid-19 vaccine booster, the frequency may be 3-5 years similar to the diphtheria vaccine because most nCoV mutations are less likely to escape the body’s immunity.

On October 30, the Department of Health of Ho Chi Minh City proposed the People’s Committee and the Ministry of Health to allow three doses of Covid-19 vaccine for high-risk groups and frontline forces to fight the epidemic in the last two months of the year. This is a booster shot for people who have had two full shots after 6 months to a year, depending on the type of vaccine, not a booster. France, Germany, the UK, the US, Singapore … have injected three doses of Covid-19 vaccines.

To date, more than 7.6 million people in Ho Chi Minh City have received the first dose of the Covid-19 vaccine, and more than 5.7 million people have received the second dose.

Cam Le – Chile


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