‘Time right to boost immunity’
An in-depth look at immunity is given by Consultant Immunologist,
Dr. Rajiva de Silva, of the Medical Research Institute (MRI), Colombo.
The four main vaccines of AstraZeneca (AZ); Sinopharm;
Pfizer and Moderna, were administered in Sri Lanka originally, with a small group being given Sputnik V, he says, going back to early 2021 (last year).
Dr. de Silva is very specific – vaccines may not prevent a person from getting infected by the COVID
19 virus, whether it is the original Wuhan virus or subsequent variants and subvariants. But they do ward off severe disease.
Stressing that boosters ‘boost immunity’ (antibody and cell mediated response) that is already present, he says that after the first booster (sometimes people in Sri Lanka call it the 3rd dose), the second booster (4th dose) should be taken around six months later. So the time is right for people to take the booster now, especially those over-60 and those with co-morbidities.
He explains that the same happens if people get a natural infection. Next he focuses on ‘antibodies’, ‘memory B cells’ and the ‘cell-mediated immune response by T cells’.
A person’s response when an antigen (either the virus or the vaccine) enters the body:
· ‘Memory B cells’ get activated if a person has got the vaccine or a prior COVID-19 infection. These transform themselves to either ‘plasma cells’ or produce more memory cells. These ‘plasma cells’ produce great quantities of ‘antibodies’. The ‘antibodies’ block the virus from entering the cells of the respiratory tract.
· ‘Memory T cells’ will also get activated and some of the ‘sub-sets’ of these cells (cytotoxic T cells) will kill virus-infected cells. Even if there are no effective ‘antibodies’, the ‘T cells’ can kill the virus.
“Antibody responses diminish over time and they may relatively be less effective against Omicron’s sub-variant BA.4 and BA.5. However, T cell responses are relatively well preserved, a study on Omicron’s sub-variant BA.1 conducted in the United States of America (USA) and published in the high-impact medical journal ‘Cell’ has shown,” he says, adding that this is while a sub-set of study participants had shown a reduction in the T cell response.
He explains that this is why it is likely that although vaccinated or previously infected persons, may still develop COVID-19 due to BA.4 or BA.5. But the severity would be reduced due to the presence of the T cell response.
While conceding that there is not much information on BA.4 and BA.5, Dr. de Silva says that these sub-variants are more transmissible than the original Omicron sub-variant BA.1. The main reason for an increase in infections recently is because BA.4 and BA.5 partially evade antibody responses evoked by previous COVID-19 infections and vaccination.
Meanwhile, citing a recent study on the 3rd and 4th doses conducted by the US Centres for Disease Control (CDC), Dr. de Silva adds that vaccine effectiveness against hospitalization for BA.2 at 120 days (around four months) after three doses, in those over-50 years of age, was 55%. The effectiveness rose to 80%, seven days after the 4th dose. This is why after four to five months, there is a need to boost immunity.