As defined by the CDC, “booster doses” are vaccine doses after primary (1 or 2-dose) series that are needed to increase immunity after waning of the initial immune response. For some individuals, sufficient immune response may not be produced after primary dose series and hence, require additional dose to reach protective immunity.¹

Also, the delta variant and its virulence are making many countries throughout the world start administering booster vaccines. Studies have reported that a single dose of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccine is only around 30% effective against delta, whereas the second dose provides protection of about 88% for Pfizer and 67% for AstraZeneca. Mostly the high and middle-income countries are planning for administering booster doses to ensure the safety and protection of people against the new variants of SARS-CoV-2. Almost more than nine million booster doses had been administered worldwide by 18 August. ²

Though around 35 million covid-19 vaccines doses are being administered every day, yet only 1.3% of people have received at least one dose in low-income countries. Overall, just 32% i.e one-third of the world’s population has received at least one dose, and only a quarter (24%) has received the second dose.²

Germany and Israel have announced plans for booster-shot programmers, while other countries like the United Arab Emirates, China and Russia have already initiated administering extra doses. The World Health Organization called for a moratorium on boosters and stated that administering booster doses to those who are already protected was not sensible.³

Hence, many questions are still running in the minds of all and still are not clear to scientists whether most people need the booster dose or not . Some important questions have emerged as the discussion moves on.

Do boosters actually work?

Vaccination produces an initial increase in the number of immune cells which enhances antibody production. But, slowly the number decreases leaving behind a small pool of long-lasting ‘memory’ B and T cells that aid in the protection of the body from future infection from the particular pathogen.

As suggested by some immunologists, a booster dose results in the multiplication of antibody-making B cells elevates the antibody levels against the particular pathogen again. With time their numbers would reduce again, but the pool of memory B cells left behind would be larger than before, leading to a faster, stronger response to subsequent exposures. Booster doses provide affinity maturation; where the B cells triggered by the vaccine travel to the lymph nodes and gain mutations and make the antibodies produced by them bind to the pathogens more strongly, thus enhancing their potency.

Few trial studies showed that the third doses of vaccines developed by Moderna, Pfizer–BioNTech, Oxford–AstraZeneca and Sinovac increased the levels of ‘neutralizing’ antibodies when administered several months after the second dose. In another study in the UK, various combinations of boosters would result in more robust immune responses, characterized by high levels of both antibodies and T cells. Some immunologists are of the view that a booster dose would show a stronger immune response

Is immunity from vaccines waning?

To answer this question, scientists study the antibody levels and titres, as a proxy for how effective a vaccine has worked. These usually spike along with the surge in short-lived B cells and then fall as the cells dwindle. Memory B cells and bone marrow plasma cells continue to produce antibodies, but at low levels, for a long time. Though with time, antibody levels fall, but the question remains whether the decline in antibody level reduces the protection also. So, worldwide studies are being carried out to determine the optimum level of neutralizing antibodies or another immune marker that can unearth the effectiveness of vaccines more closely. They’re seeking what’s known as a correlate of protection.

Are vaccinations given months ago still preventing infections?

Raw data on vaccinations and infections from December 2020 to July 2021released by the Ministry of Health in Israel estimated that vaccine protection against both infection and disease had decreased from 90% in the months of initiation of vaccine program to about 40% by the end of June. This decline in protection could be due to the effects of the Delta variant.

In, another study, the scientists from Kahn Sagol Maccabi in Tel Aviv reviewed the health records of more than 1.3 million vaccinated people between January and April 2021. It was observed that among those vaccinated in January and February there was a chance of 53% of people, who might test positive for SARS-CoV-2 during those four months, as compared to those people who were vaccinated in March and April. The reduced protection could also be due to age differences, workplace differences e.g healthcare workers, were at a higher risk of infection than the younger people vaccinated later, stress caused due to the pandemic, more eagerness to travel internationally etc.

From the data published on the preprint server medRxiv on 28 July, it was reported that the Pfizer–BioNTech vaccine’s efficacy against symptomatic disease had slipped from 96% to 84% after 6 months. Previous to this release, an April press release from Moderna claimed and reported that the Moderna vaccine was effective at 90% after half a year, compared to its original efficacy of 94%.³

But Pfizer and other vaccine-makers unblinded the studies, once these vaccines became publicly available. Thus the people could know whether they had received a vaccine or a placebo. All the participants who learn that they had been vaccinated and were 95% protected felt safer and started taking more risks and not following restrictions, which might also have resulted in the decline in the efficacy of the Pfizer–BioNTech vaccine. Though Pfizer had no comment on this hypothesis.³

Thus we see that, though some of the developed countries have already started introducing the booster doses of COVID-19, still availability of the mere first dose remains a question to majority of worlds’ population in lower, middle and developing countries.

References:

1. COVID-19 Vaccine Booster Shot: National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases; 2021 [updated 1-Sep-21; cited 2021 13-Sep-21]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/booster-shot.html.
2. Mahase E. Covid-19 booster vaccines: What we know and who’s doing what. BMJ. 2021;374:n2082.
3. COVID vaccine boosters: the most important questions, Nature,  596, 2021, 178-180 , Available from: COVID vaccine boosters: the most important questions (nature.com)