Wednesday, Oct 05, 2022

Explained: What three new studies say about Covid-19 vaccine efficacy, booster shots

The studies were released about a week after the United States Food and Drug Administration authorised booster shots for certain categories of people.

covid vaccine booster shots explainedA health worker administers a Covid-19 vaccine amid the pandemic (File Photo)

Three new studies published by the US Centers for Disease Control and Prevention (CDC) say that the protection offered by vaccines Pfizer and Moderna against SARS-CoV-2 wanes over time, prompting a debate among experts about the need for booster shots for everyone eventually. Even so, these vaccines have been largely effective against hospitalisations, the studies have found.

Significance of the new studies

The studies were released just about a week after the Food and Drug Administration (FDA) authorised booster shots (which means a third shot of the vaccine) for transplant recipients and people with weakened immune systems to better protect them from variants of Covid-19.

The FDA said those who had received either the Moderna or Pfizer-BioNTech vaccines could take a third dose. There is no information on beneficiaries of the Johnson and Johnson single-dose vaccine yet, but recipients of this vaccine will likely require booster shots.

Countries such as Israel, Germany and France have already laid out plans to roll out the administration of booster shots.

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Meanwhile, the World Health Organisation has placed a moratorium on Covid-19 boosters in August due to the disparity in vaccination levels in low and high-income countries. The WHO’s Director-General, Tedros Adhanom Ghebreyesus, emphasised previously that countries that have already used up a major chunk of the global vaccine supply cannot go on using more of it.

What the studies have found

One of the studies was conducted between May 3 and July 25 and has found that while the overall age-adjusted vaccine effectiveness against hospitalisations in New York remained relatively stable (between 91.9-95.3 percent), the overall vaccine effectiveness against infection for all age-groups of adults in New York was reduced from 91.7 per cent to 79.8 per cent.

These results are from the time when the circulation of the Delta variant in the US was high. One reason why the effectiveness of vaccines is reduced against infections could be because of the increased viral load associated with the Delta variant.


Another study conducted between March and July found that among 1,129 patients who received two doses of one of the mRNA vaccines, no decline in vaccine effectiveness against Covid-19 hospitalisations was observed after six months. This study found that the vaccine effectiveness was 86 per cent 2–12 weeks after vaccination and 84 per cent at 13–24 weeks. Vaccine effectiveness was sustained among groups at risk for severe Covid-19.

“Protection against severe Covid-19 resulting in hospitalization was sustained through 24 weeks after vaccination with mRNA COVID-19 vaccines,” the study said.

Another study said that two doses of mRNA vaccines were 74.7 per cent effective against infection among nursing home residents between March-May. This study says that during June-July, when the Delta variant was predominant, the vaccine effectiveness declined to 53.1 per cent. “An additional dose of COVID-19 vaccine might be considered for nursing home and long-term care facility residents to optimise a protective immune response,” this study has said.


Who can get booster shots in the US?

As of now, the CDC has recommended booster shots for those who are moderately or severely immunocompromised because they are believed to be more at risk of serious, prolonged illness from Covid-19.

This is because persons with weakened immune systems may not build the same level of immunity against Covid-19 after a two-dose regimen relative to those who are not immunocompromised. Therefore, for now, immunocompromised people in the US can receive a third dose of the vaccine at least 28 days after their second dose was delivered.

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First published on: 19-08-2021 at 07:05:03 pm
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