A vaccine booster or a 3rd dose, what is the difference? What is the best way forward in this regard, considering research carried out across the world as well as in Sri Lanka? These are the questions and much more on immunity that Consultant Immunologist, Dr. Rajiva de Silva, attached to the Medical Research Institute [...]


Vaccines and immunity under the microscope

Dr. Rajiva de Silva looks closely at effectiveness of jabs, boosters, 3rd doses, antibodies, B cells and T cells

A vaccine booster or a 3rd dose, what is the difference? What is the best way forward in this regard, considering research carried out across the world as well as in Sri Lanka?

These are the questions and much more on immunity that Consultant Immunologist, Dr. Rajiva de Silva, attached to the Medical Research Institute (MRI), Colombo, sheds light on, in an interview with the Sunday Times this week.

Sri Lanka has vaccinated its population mainly with four vaccines – AstraZeneca (AZ); Sinopharm; Pfizer and Moderna, with a small group being administered Sputnik V.

Dr. Rajiva de Silva

Looking at the four main vaccines, Dr. de Silva says that the impact of different vaccines on different age groups varies. Studies carried out in Bahrain some time ago with regard to Pfizer, AZ, Sinopharm and Sputnik V had found that there were more hospitalizations and deaths among the unvaccinated than the vaccinated.

From among the vaccines themselves, it had been recorded that there were a few more hospitalizations and deaths among people over-50 years of age who had got the Sinopharm jab, than others who had been administered the Pfizer, AZ or Sputnik V, he says.

With regard to effectiveness against severe disease and death, he explains that in the same age group, AZ and Pfizer were “very good” up to 5-6 months. Data from several countries available up to 6 months indicate that even though people who had got AZ and Pfizer were vulnerable to more disease as time passed and closer to 6 months, the protection against severe disease and death was “stable” up to 6 months. Sinopharm too gave protection, but this cover was less in the over-50s, than from Pfizer and AZ.

Picking up the Delta variant which has caused havoc across the world including Sri Lanka, Dr. de Silva says that though mild disease increased even after the Pfizer, AZ and Sinopharm jabs, there was minimal severe disease, hospitalization and death among the vaccinated, compared to the unvaccinated. The people who were vaccinated but got severe disease and died had co-morbidities or were in the elderly group.

Delving into the reasons for giving a Pfizer booster to those over-60 years of age, this Immunologist looks at Israeli studies. They had found that in those who got a booster on top of two doses of Pfizer, 5-6 months after the two earlier doses, the infection rate went down, compared to those who did not get the booster. The booster in the over-60s led to a “significant” reduction in severe COVID-19 and also an 11-fold reduction in infection. However, the observation period of this study had been of a short duration.

Quoting local data from Prof. Neelika Malavige of the University of Sri Jayewardenepura, he says that it had been found that about 7% of people over-60 years of age who had got both doses of Sinopharm did not have antibodies against the infection. This was why a decision was made to give all those over-60 years of age a 3rd dose.

Difference between a booster and a 3rd dose

Dr. de Silva says that the difference between a ‘booster’ and a ‘3rd dose’ is that a booster ‘boosts the immunity’ (antibody and cell mediated response) that is already present. The booster is given around six months after the 2nd dose.

A 3rd dose, meanwhile, in a primary series (after the initial two doses) is given to people with a weakened immune system to produce an adequate immune response. The 3rd dose should be given at least a month after the 2nd dose, he says.

People with a weakened immune system are those after organ/stem cell transplantation, those on dialysis, those on immuno-suppressive drugs, those on chemotherapy for malignancies and those who have advanced HIV/AIDS).

Why is Sri Lanka administering Pfizer as a booster?

Dr. de Silva says that both AZ and Pfizer vaccines lead to antibody production in those vaccinated.

The ‘memory B cells’ produced by the AZ vaccine also recognize a similar epitope (part of the vaccine that the vaccinated person’s immune system recognizes) in the Pfizer vaccine. Then the ‘memory B cells’ get activated, produce more ‘memory B cells’ and also more ‘antibodies’.

“Many studies in other countries on ‘mixing and matching’ vaccines have found that it is better to mix the primary immunization (the initial two doses). This means that a ‘mix’ of one dose of AZ with a second dose of an mRNA vaccine (Pfizer or Moderna) would give a better immune response. In addition, it also broadens the immune response,” explains Dr. de Silva.

Citing an example, he says that AZ produces a stronger ‘cell-mediated immune response’ compared to Pfizer, whereas Pfizer produces a better ‘antibody’ response. Therefore, mixing these two vaccines would broaden the immune response.


Dr. de Silva reassures people who may not have produced an ‘antibody’ response to vaccines that it does not mean that they are not protected as there are other mechanisms (such as ‘memory B cells’ and the ‘cell-mediated immune response by T cells’) at work as well.

“So when an antigen (either the virus or the vaccine) comes into the body, these ‘memory B cells’ get activated and transform themselves to either ‘plasma cells’ or produce more memory cells. These ‘plasma cells’ produce great quantities of ‘antibodies’ which are more effective than those produced in the primary response,” he explains.

These ‘antibodies’ block the virus from entering the cells of the respiratory tract, preventing disease. In addition, ‘memory T cells’ will also get activated and some of the ‘subsets’ of these cells (cytotoxic T cells) will kill virus-infected cells. So, even if there are no ‘antibodies’, the ‘T cells’ can kill the virus.

Delta Plus variant

“No one knows the exact implications of Delta Plus. It has not yet been classified as a ‘variant of concern’ or ‘variant under investigation’ by the World Health Organization (WHO). We don’t know much about it,” said Dr. Rajiva de Silva.

When asked whether vaccines will give protection against Delta Plus, he said that probably it will provide protection, but the extent of the protection is unknown.

 AMS warns against irresponsible behaviour and COVID-19 resurgence

Will there be a rebound of COVID-19 following the relaxation of restrictions, asks the Association of Medical Specialists (AMS), referring to the “irresponsible” and “complacent” attitude of some groups of the public.

“If we continue to behave irresponsibly in this way, another wave of COVID-19 will be inevitable and will be the last nail on the coffin of an already battered economy. Such behaviour not only undermines what we have already achieved but also sends wrong signals to the rest of the public to follow suit,” warns the AMS, adding that it also disheartens civic-minded society who can sense the threat of another surge.

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