Phạm Quang Thái, head of the northern regional office of the National Expanded Programme on Immunisation (NEPI). Photo suckhoedoisong.vn

Việt Nam has recorded new cases of mild Omicron BA.5 variant and experts are worried it could become more and more predominant. To better understand the effectiveness of vaccination against COVID-19, Phạm Quang Thái, head of the northern regional office of the National Expanded Programme on Immunisation (NEPI) talks to Sức khoẻ&Đời Sống (Health and Life) online newspaper about the issue.

How long must one wait to get the third and fourth shots of COVID-19 vaccine after getting the second dose? If the scheduled vaccination date is coming up and you have COVID-19, should you get the vaccine and how should it be given?

To avoid confusion when it comes to booster shots, we separate into basic doses (1, 2 or 3 doses depending on the type of vaccine and subject to injection) and booster doses. The third dose of COVID-19 vaccine is understood as the first booster shot. The fourth dose is the second booster shot.

The first and second booster shots of COVID-19 vaccine (equivalent to the third and fourth injections or fourth and fifth injections depending on the subject) help maintain a high level of immunity for adults who have received the full basic doses.

The first booster shot and second booster shot can be used with the same injectable vaccine as the basic dose, or with mRNA vaccine or AstraZeneca vaccine if given in combination. It should be noted that the interval between this first booster injection and the last basic injection is at least three months. The second booster shot must be four months after the first booster shot. Particularly for the second booster, it should be three months from the point of time that one has COVID-19. This is not the case with the first booster shot.

The booster dose for Moderna vaccine is 0.25ml (half of the basic dose); for other types of vaccines, same as basic doses according to manufacturer's instructions.

For the elderly and people with certain medical conditions, is there anything they should do differently?

With the current situation of COVID-19 pandemic in Việt Nam and the implementation of the third dose injection reaching a rate of over 65 per cent in people aged 18 years and older, facing the risk of a new pandemic wave, it is very necessary to administer second booster injections for at-risk group of people to reduce the risk.

Currently, many people are confused between the additional dose of COVID-19 vaccine and the third dose (first booster). Additional vaccines are recommended for immunocompromised subjects or for those receiving vaccines where evidence suggests that additional vaccination is required to achieve baseline immunity (eg, Vero Cell vaccine, Sputnik V).

Particularly with the second booster injection (shot number four), it will be especially useful for people who are 50 years of age and older, people aged 18 years and older with moderate to severe immunodeficiency, people aged 18 and over are in high-risk groups of exposure to COVID-19 such as health workers, frontline officials (police force, army, teachers, people working in the transportation sector, etc.), people providing essential services, people working at tourist service establishments, trade centres, supermarkets, wet markets, workers, people working in industrial parks.

The guidance in the latest document of the Ministry of Health states that the injection of an additional dose (this dose is not the third dose) is for those of 18 years old or older, and having moderate and severe immunodeficiency such as adults receiving active cancer treatment for tumours or blood cancers; organ transplant recipients who are taking immunosuppressive drugs; people receiving T-cell antibody receptor therapy (a type of treatment that helps the immune system attack and destroy cancer cells) or have a stem cell transplant (within the past two years); people with moderate or severe primary immunodeficiency (eg, DiGeorge syndrome, Wiskott-Aldrich syndrome); people with advanced or untreated HIV; People who are on active treatment with corticosteroids or high-dose immunosuppressive drugs; and people who have received the full basic dose of Sinopharm vaccine (Vero cell) or Sputnik V vaccine.

Many people think that they have been vaccinated against COVID-19 with the first, second and third doses, plus the fact that they have been infected with COVID-19 recently, so they do not need the fourth dose of the vaccine. What are your thoughts on this?

The COVID-19 treatment management system of the Ministry of Health has recorded more than 40,000 deaths, most of these have not been vaccinated against COVID-19. A small proportion of these deaths are those who had received one or two doses of vaccine but have not received the third dose. This data once again confirms the need for a first booster shot for those who have completed the basic injections.

In theory, after recovering from COVID-19, the body will have antibodies to fight the virus that causes the disease. However, the reality is that not everyone has good enough antibodies because the infection with the virus is different depending on the strain, the infection situation, the degree of infection and the characteristics of the body.

There are quite a few cases of infection and re-infection because the antibodies produced are not good enough or the first infection is only transient and then the infection becomes severe again.

In addition, scientific evidence shows that when one had been infected, then vaccinated, the protective antibodies produced will be much higher and contribute to limiting reinfection as well as limiting post-COVID conditions. This is the reason why even though the patient has recovered, the patient is still advised to continue the vaccination to complete the injection regimen.

The third and fourth doses of COVID-19 vaccine will restore immunity, maintain protection against the risk of SARS-COV-2 virus infection because among people who have received the basic doses and have COVID-19 infection, immunity starts to decrease at 10-19 weeks post-injection, or about three to four months after the injections.

The results of a study in France showed that if given a booster dose, these people were protected from re-infection by up to 81 per cent. Research results in the United States show that among people who have had COVID-19 with only two doses, the effectiveness of preventing re-infection is 34.6 per cent, but if given a booster dose, this effect figure is up to 67 ,6 per cent.

For children over 12 years old, is there any difference when getting the third dose of COVID-19 vaccine?

According to the document by the Minister of Health, the booster dose for children aged 12-17 will be Pfizer shots (Comirnaty), and will be administered five months after the second dose.

The dosage is 0.3ml, the same as the standard dose in the primary course for this age group.

Booster shots can be delayed by three months after a COVID-19 infection. VNS