As has been shown in other countries, vaccine boosters performed better against the Delta variant than Omicron, a highly mutated version of the SARS-CoV-2 virus that has been able to evade immunity from vaccines and prior infections.
One of the studies, published on Friday in the CDC’s Morbidity and Mortality Weekly Report, looked at rates of hospitalisation, emergency department and urgent care visits in 10 states between Aug 26, 2021, and Jan 5, 2022.
It found that protection from two doses of vaccine fell to 57 per cent in people who got their second shot at least six months earlier. Among those who received a booster, protection from hospitalisation and urgent care visits was 90 per cent.
In another study published in the JAMA medical journal, researchers reviewed data on 23,391 COVID-19 cases caused by either the Delta or Omicron variant among people seeking testing between Dec 10, 2021, and Jan 1, 2022.
They found that among people seeking testing for COVID-like symptoms, those who had received three doses of an mRNA vaccine had the highest protection from infection compared with those who got two doses or were unvaccinated.
Because of the timing of US booster recommendations, most people in the study had been vaccinated within a month of seeking testing, which likely contributed to the benefit.
Data from the UK has shown that the increased antibody protection gained from boosters wanes after 10 months, so it is unclear how long that benefit would hold.
Some countries have already started offering additional booster doses, but a recent study from Israel showed that while a fourth dose of an mRNA vaccine boosted antibodies, the level was not high enough to prevent infection by the Omicron variant.
Early enthusiasm for boosters in the United States has been lacklustre, fuelled in part by rapidly shifting public health messaging and concerns among some experts over a lack of US data demonstrating their benefit.
There is also the misguided perception among some Americans that if you can still get infected, why bother with a booster?
According to the CDC, only 82.5 million, or 39.3 per cent, of fully vaccinated Americans have received a COVID-19 booster dose.
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