Is your understanding of the “mixed” COVID-19 vaccine correct?

Xinhua News Agency, Beijing, February 21 (Reporter Li Wen) China has recently begun to deploy sequential booster vaccination of the new crown vaccine. So, does sequential vaccination mean "mixing" different brands of vaccines? In fact, sequential vaccination emphasizes the interval vaccination of vaccines with different technical routes. So why do we need sequential vaccination? Are there similar deployments abroad?

Why sequential vaccination?

Sequential vaccination refers to the intermittent vaccination (alternating vaccination) of vaccines with different technical routes. Shao Yiming, a member of the expert group of the vaccine research and development team of the joint prevention and control mechanism of the State Council of China, said at a press conference of the joint prevention and control mechanism on the 19th that for viruses that are highly variable and difficult to deal with, sequential immunization is often used.

Wang Huaqing, chief expert of the immunization program at the Chinese Center for Disease Control and Prevention, said that some countries in the Americas, Europe and Southeast Asia have begun to implement sequential booster vaccinations.

The European Medicines Agency and the European Centre for Disease Prevention and Control issued a statement in December 2021 saying that multiple studies have shown that mixed vaccination ("mixed") of adenovirus vector vaccines and mRNA (messenger RNA) vaccines can "produce better antibody levels" and "produce better T cell immune responses compared to vaccines using the same technology," regardless of whether full vaccination or booster shots are given. T cells are one of the body's lines of defense against disease.

The statement said that "mixed vaccination" can "provide more vaccination options, especially when a certain vaccine cannot be administered for some reason, it can reduce the impact on vaccine promotion."

Shao Yiming introduced that there are two main purposes of using sequential booster immunization: one is that different vaccines can complement each other's advantages, and the other is that different people's physiques may produce more side effects to a certain type of vaccine. Using different technical routes for vaccination can avoid side effects.

Temporary Guidelines for Mixed Fights

Based on existing research data, the World Health Organization issued interim guidelines for mixed vaccination of new coronavirus vaccines in December 2021.

The guidelines are based on the recommendations of the WHO Strategic Advisory Group of Experts and apply to all COVID-19 vaccines on the WHO Emergency Use List. The guidelines suggest that if the first dose is a viral vector vaccine, the second dose or booster shot can be followed by an mRNA vaccine, and vice versa. If the inactivated vaccine is initially administered, a viral vector vaccine or an mRNA vaccine can be administered later.

So far, the vaccines on the WHO emergency use list include: AstraZeneca vaccine from the UK, Johnson & Johnson vaccine, Moderna vaccine and Pfizer vaccine from the US, Sinopharm vaccine and Sinovac vaccine from China, Covaxin vaccine and Covovax vaccine from India, and Novavax vaccine from the US. Among them, Sinovac, Sinopharm and Covaxin are inactivated vaccines, Johnson & Johnson and AstraZeneca are adenovirus vector vaccines, Moderna and Pfizer are mRNA vaccines, and Novavax vaccine and Covovax are recombinant protein vaccines.

According to the interim guidelines, existing research mainly involves the impact of "mixed shots" on human immunity. It is still the current standard practice to choose the same vaccine as the first shot for the second and third shots, but as research deepens, WHO will re-evaluate the advantages and risks of "mixed shots". As more research data comes out, it is not ruled out that the "mixed shots" guidelines will be adjusted.

Many countries approve "mixed fighting"

On October 20, 2021, the U.S. Food and Drug Administration revised the emergency use authorization for the COVID-19 vaccine, allowing recipients to choose a vaccine booster shot that is different from the brand of vaccine they previously received. Peter Marks, director of the U.S. Food and Drug Administration's Center for Biologics Evaluation and Research, said the decision to allow "mixed" booster shots is intended to meet the needs of the public as much as possible.

After studying the results of the mixed COVID-19 vaccine trials in Spain and the United Kingdom, Canada's National Advisory Committee on Immunization announced new regulations in June 2021 to allow mixed COVID-19 vaccines in order to further speed up the vaccination process. People who receive the AstraZeneca vaccine for the first dose can choose the same vaccine for the second dose, or they can choose the Pfizer or Moderna vaccine.

South Korea, the United Kingdom and other countries have launched clinical trials on sequential vaccination of COVID-19 vaccines. South Korea's disease control department launched a clinical trial of sequential vaccination in May 2021. The first batch of test subjects were about 500 people who had received one dose of AstraZeneca vaccine, and the second dose was Pfizer and other COVID-19 vaccines. The test results showed that people who received AstraZeneca and Pfizer vaccines sequentially had a 6-fold increase in neutralizing antibody levels compared to those who received two doses of AstraZeneca vaccine.

The University of Oxford in the UK launched a clinical trial of sequential vaccination of the new crown vaccine in February 2021. The first phase of the clinical trial used a mixture of AstraZeneca vaccine and Pfizer vaccine, and the second phase of the trial conducted in April used Moderna vaccine and Novavax vaccine.

The test results show that the antibody levels produced by volunteers who received the Pfizer vaccine first and then the AstraZeneca vaccine were about 5 times that of volunteers who received two doses of the AstraZeneca vaccine, and the antibody levels produced by volunteers who received the AstraZeneca vaccine first and then the Pfizer vaccine were 9 times higher than those who received two doses of the AstraZeneca vaccine. The antibody levels produced by volunteers who received the AstraZeneca vaccine first and then the Moderna or Novavax vaccine will be higher than those who received two doses of the AstraZeneca vaccine.

Source: Xinhuanet