Updated Aug 2022
Are COVID vaccine boosters required?
There is still no conclusive evidence regarding the actual clinical benefit of boosters of the currently existing COVID vaccines. As of now evidence from the clinical trials and real-world evidence of different COVID vaccines confirms that these vaccines produce neutralizing antibodies that are protective against SARS-CoV-2 antigens, and primarily help to prevent severe COVID or resulting complications, hospitalization, and death.
While neutralizing antibodies are what constitute the ‘humoral immunity’, there is also a ‘cell-mediated immunity’ that develops post-vaccination as a result of a response mediated by the T lymphocytes (white blood cells that act directly and have memory). While the neutralizing antibodies component can be quantified and has seen to decrease over time especially beyond 6-9 months, it is not possible to estimate the duration of T-cell immunity that is usually more long-lasting and protective especially in the case of viruses.
However, booster vaccine shots are now being given in many countries, that have achieved a high level of completion of primary 2-dose immunization against COVID of the eligible population, and are have also seen a resurgence of COVID due to one or the other variants. Therefore, if one is residing in such countries, one may receive a booster shot as part of the national administrative policy.
Who should receive the COVID booster?
The WHO recommends that the introduction of booster doses in individual countries should be based on the primary COVID vaccination schedule status, age and/or underlying medical conditions of the vaccine recipient, risk of exposure, and circulation of specific variants. The decision to recommend a booster dose is complex and requires clinical and epidemiological data, as well as consideration of national strategy and vaccine supply. Blanket booster doses from a global perspective can create vaccine inequality and ironically increase virus spread and mutation in countries yet to complete primary 2-dose vaccination in large sections of the population.
Some countries that have adequate or surplus vaccine stocks are giving a booster shot to all the people who have completed their primary 2-dose immunization 6-9 months back or more. People who have ‘hybrid immunity’ that is from both the vaccination and having suffered from COVID, are likely to be better protected and therefore in less requirement of the booster.
However, there are certain groups of people who are at higher risk of getting infected or suffering from severe COVID and its complications. These groups should be prioritized for boosters and include those who:
- have been receiving active cancer treatment
- had an organ transplant and are taking immunosuppressive medicines
- have immune deficient or immune-suppressive conditions (primary immunodeficiencies or secondary conditions like HIV)
- are taking drugs like high-dose corticosteroids or other medications that may cause suppression of the immune system
- have comorbidities like heart failure, cardiovascular disease, diabetes, hypertension, kidney or liver disease, and lung conditions like COPD, etc.
- are elderly > 70 years
- are healthcare workers or frontline workers at higher risk of COVID exposure and infection.
Which vaccines are eligible to be given as booster shots?
As recommended by the CDC, the mRNA vaccines of PfizerBioNtech (BNT162b2) and Moderna (mRNA-1273) are approved as booster shots in those over 16 and 18 years respectively. The adenoviral Janssen/Johnson & Johnson single-dose vaccine (Ad26.CoV2.S) is also approved for a booster shot in those 18 years or more.
Recommendations have also come in for a booster shot for the adenoviral vector vaccine AZD1222 Astra Zeneca in EU. In India booster shots (called precautionary dose) with Covishield and whole virus killed vaccine Covaxin have started initially for health/front line workers as well as for elderly and those with comorbidities, and now for all adults
The booster shot could be of the same vaccine (homologous booster) or of a different vaccine than the primary shot (heterologous booster), with the latter showing better antibody response in studies. Heterologous booster shot is permitted in the US and EU. India has approved its vaccine Corbevax for heterologous booster shots.
What is the timing of the booster vaccine shot?
The booster shot with mRNA vaccines or adenoviral vector vaccine AZD1222 is recommended after 6 months have elapsed from the 2nd dose of one’s primary 2-dose COVID vaccination. As of now, a single booster shot is being given in the countries which have started the same. In the case of people with immune deficiencies, the booster can be given as early as 28 days after the 2nd dose.
For the single dose Janssen/Johnson & Johnson, the booster is given after 2 months have elapsed post the primary first dose. In India booster shots with Covishield and Covaxin are currently being given 9 months or more after the primary 2nd dose.
Does the booster vaccine protect from variant strains?
Most of the vaccines are based on the spike protein S antigen which aids viral entry into human cells. However, the variant strains show mutations in the S protein, and therefore this may reduce the effectiveness of the current vaccines. Breakthrough COVID infections have been seen in fully vaccinated people during the 2nd wave with the Delta variant, as well as with the recent Omicron variant.
Therefore, the ideal boosters will be ‘updated vaccines’ incorporating the S antigen mutations of the variant strains. However, till such updated vaccines are developed and made available, primary 2-dose vaccination and a booster with the currently available COVID vaccines remain the available so recommended way of protecting oneself from severe COVID and its complications and hospitalization, even with respect to variant strains.