Covid-19 vaccine booster shots: what do we know so far?
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Covid-19 vaccine booster shots: what do we know so far?

By Darcy Jimenez 25 Aug 2021 (Last Updated November 15th, 2021 11:37)

A number of countries have begun rolling out Covid-19 vaccine booster shots to bolster protection against the virus in certain populations.

Covid-19 vaccine booster shots: what do we know so far?
US health officials say “boosters will likely be needed” for individuals who received Johnson & Johnson’s one-shot vaccine. Credit: Shutterstock.com

In August, US officials announced that from September, double-vaccinated adults aged 18 and older would be offered Covid-19 vaccine booster shots, pending data to confirm the safety and efficacy of a third dose of either the Pfizer or Moderna jab.

The decision was made in response to evidence that protection against coronavirus is decreasing, which public health officials noted is likely due to “both waning immunity and the strength of the widespread Delta variant”.

Booster shots: what’s the evidence?

While data shows that two vaccine shots are largely effective at protecting against severe Covid-19, data on the benefit of booster shots is currently unclear. To establish whether a booster dose would offer significantly increased protection, experts first need to understand the level of antibodies and other immune defences needed to prevent severe illness and death from Covid-19. Once this has been measured, and immunity is seen dropping to this level, a case for routinely administering booster shots can be made.

A recent Israeli study found a third dose of Pfizer’s vaccine to be 86% effective in people aged over 60, and a UK trial assessing whether a third shot of different Covid-19 vaccines could boost immunity against the virus is ongoing, with results expected in September.

Preliminary data from an Oxford University-led study evaluating the AstraZeneca vaccine has also found that third dose of the jab boosts antibodies against Covid-19.

What are countries’ plans for booster shots?

The FDA granted emergency use authorisation for a single booster dose of Pfizer’s vaccine, in individuals aged 65 and over and those at risk of severe Covid-19, in September. Healthcare workers and others at high risk for occupational exposure are also included in the approval, but the use of boosters for the general population is yet to be permitted. Booster shots will be administered at least six months after the second vaccine dose, the FDA said.

In October, the FDA also gave emergency use authorisation for single booster doses of both the Moderna and Johnson & Johnson Covid-19 vaccines. The additional Moderna dose, to be administered six months after the first two, is approved for those aged 65 and older, those 18 to 64 years old at high risk of severe Covid-19, and those aged 18 to 64 years “with frequent institutional or occupational exposure” to the virus.

The Johnson & Johnson booster can be administered to individuals aged 18 and older, two months after completion of their first dose of the single-shot jab.

The agency has also permitted a mix-and-match approach to booster shots, meaning eligible individuals can receive a booster dose of an approved vaccine that differs from the one used for their primary vaccination.

UK health regulator MHRA has approved the AstraZeneca and Pfizer vaccines as safe and effective for third shots, and the Joint Committee on Vaccination and Immunisation (JCVI) has recommended that boosters should be offered to all over-50s and people at severe risk of illness from Covid-19.

In line with advice from the JCVI, the NHS began offering third booster doses to at-risk populations in mid-September. Those who are eligible for booster shots in the UK include: those living in residential care homes for older adults, everyone aged 50 years or over, frontline health and social care staff, those aged 16 to 49 with underlying health conditions putting them at higher risk of severe Covid-19 (as set out in the green book), and adults living with immunosuppressed individuals.

In mid-November, the JCVI also recommended that a third vaccine dose be offered to everyone aged over 40 in the UK. People between 40 and 49 would receive boosters of either the Pfizer jab or half a dose of the Moderna vaccine, six months after their second vaccine dose.

In early October, the European Medicines Agency approved Moderna’s Covid-19 booster shot for immunocompromised individuals aged 12 years and above.

Israel has already kicked off a booster shot programme for older members of the population, while both France and Germany plan to offer third doses to the elderly and vulnerable from September.

The World Health Organization, however, has said booster vaccination schemes will “exacerbate inequities by driving up demand and consuming scarce supply”. Introducing third vaccine doses should be “firmly evidence-driven and targeted to the population groups in greatest need”, the organisation said.