The results suggest that booster shots can be effective in warding off SARS-CoV-2, the researchers said. (Representational Image)
COVID-19 vaccines by Moderna and Pfizer provide protection against multiple variants of the SARS-CoV-2 virus, including the highly infectious Delta variant, according to a study.
The findings, published in the journal Nature on Tuesday, also show that those infected with the virus prior to vaccination exhibit a more robust immune response to all variants than those who were uninfected and fully vaccinated.
The results come as an increase in so-called "breakthrough" infections caused by the Delta variant among vaccinated individuals continues to raise questions about whether the vaccines offer broad protection against newly arising variants.
"Vaccines induce high levels of antibodies against Delta and most variants. And two shots are better than one," said Akiko Iwasaki, a professor at the Yale University in the US.
The results suggest that booster shots can be effective in warding off SARS-CoV-2, the researchers said.
The team collected blood samples from 40 healthcare workers in the US between November, 2020 and January, 2021 before they had received vaccinations.
In the following weeks, they periodically took additional samples after the volunteers received their first and second doses of the Moderna or Pfizer-BioNTech mRNA vaccines.
The researchers then exposed the volunteers' blood samples to 16 different SARS-COV-2 variants, including the Delta variant, and then measured antibody and T cell response to each of the variants.
The researchers found evidence of enhanced immune system response in all blood samples, although the strength of response varied by variant and by individual.
The immune response to the Delta variant in the blood of all volunteers was generally robust -- and even stronger in samples collected after the individuals' second shots, they said.
The breakthrough cases attributed to the Delta variant are unlikely to arise from a failure of vaccines, Iwasaki said.
They likely stem from the extremely infectious nature of the Delta variant, which can overcome the immune defence, she said. Other studies have also shown that vaccinated individuals tend to have less severe infections.
The researchers also divided healthcare volunteers into two groups: Those who had been infected by COVID-19 prior to vaccination and those who had not.
The immune response of those infected prior to vaccination was more robust than for those who never been infected.
"Recovering from an initial infection is like getting a first vaccine shot," Iwasaki added.
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