What are the benefits of a second Covid-19 vaccine booster shot?
- Some countries have started giving fourth doses to seniors and other vulnerable people
- But health agencies are still considering whether to expand eligibility to others
But who needs to take boosters and what are the benefits? Here is what we know so far.
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Why do some people need a fourth shot?
Studies show that immunity against the virus that causes Covid-19 wanes over time, irrespective of whether it is induced through infection or vaccination.
The Omicron variant also dodges vaccines through a structural change on the spike protein, the part of the virus targeted by vaccines.
Various countries have sought to increase vaccine effectiveness against Omicron with booster shots.
However, a study in February by the US Centres for Disease Control and Prevention found that while mRNA vaccines were highly effective against both moderate and severe cases of Covid-19 for about two months after a third dose, their effectiveness dropped substantially by four months.
The study suggests that those at high risk of severe disease from the coronavirus infection may need extra booster shots.
Now the battle is on to counter another sublineage of Omicron, BA.2, which is about 30 per cent more transmissible than the original and could threaten health systems if many high-risk people are infected.
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Do we all need a fourth dose?
So far countries that are offering a fourth dose are only extending them to high-risk groups, such as health workers, people with compromised immune systems and people above a certain age. In the US that age is 50 years and over while in Britain it is 75.
Health agencies are still considering whether to expand the eligibility to others.
Last month, US CDC director Rochelle Walensky urged people aged over 50 – especially those with underlying medical conditions or aged over 65 – to get a fourth dose four months after a third, saying they are most likely to benefit from receiving an extra booster.
“There is currently no clear evidence in the EU that vaccine protection against severe disease is waning substantially in adults with normal immune systems aged 60 to 79 years,” the agencies said on Wednesday.
But if the pandemic situation changed, it might be necessary to consider a second booster dose in that age group.
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What can a fourth dose do?
The main evidence of the potential health impact of a fourth dose of an mRNA vaccine as a second booster comes from Israel.
Israel has offered the shots to all Israelis aged 60 and over, medical workers, and those who are immunosuppressed.
Earlier this week, a study conducted by the Israeli Health Ministry and several research institutions and published in the New England Journal of Medicine showed a fourth Pfizer/BioNTech Covid-19 vaccine dose significantly boosted protection against severe illness in people aged over 60.
The study covered 1.25 million people in the age group and was conducted from January to early March, when Omicron was dominant.
The researchers found that the rate of severe disease was 3.5 times higher in those who received three doses than those who had four.
A previous study of younger people in Israel found that a fourth dose of the Pfizer/BioNTech vaccine did not do much to protect young and healthy people against an Omicron infection, but it did give moderate protection from developing symptomatic illness.
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What about inactivated vaccines?
China, which relies on inactivated vaccines, has yet to approve a fourth dose.
A study by Sun Yat-sen University in Guangzhou published in February, which was not peer reviewed, found a fourth dose of the inactivated Sinopharm vaccine could boost antibodies but did not produce them effectively enough against the Omicron variant.
The study suggested that immune response could not be endlessly boosted and there would be a “turning point” after repeated vaccination.
“Strikingly, data in the current study indicated that neutralisation breadth was not further increased by the fourth dose, but even narrowed,” researchers wrote.
Gao Fu, director of Chinese Centre for Disease Control and Prevention, said in March that China would need time to evaluate the traits of the virus, the prevalence of the pandemic and the quality of vaccines, to decide whether a fourth or even fifth dose was needed.
China is trying to persuade its vast numbers of seniors, especially those above 80 years old, to have a booster. So far, only about 56 per cent of those fully vaccinated have had a third dose.
How long does protection from the fourth dose last?
The Israeli study published this week found that effectiveness against infection in the fourth week after the second booster shot was lower than that after the third vaccine dose. The protection falls drastically after eight weeks, suggesting that protection against symptomatic infection wanes quickly.
Protection against severe illness did not wane during the six weeks after the fourth shot but the authors said longer periods were needed to know for sure.
Meanwhile, vaccines trigger human bodies to produce other immune defences, including T-cells, which offer more lasting protection.
