Fully vaccinated people who catch COVID-19 appear to be less infectious and clear the virus quicker than unvaccinated people, early research suggests

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Netherlands Pfizer Vaccine Rollout
A healthcare worker in the Netherlands is given the Pfizer-BioNTech vaccine on January 6. Piroschka van de Wouw/Pool via AP
  • Fully vaccinated people who get COVID-19 may be less infectious than unvaccinated people, a study suggests.

  • Scientists looked at 161 "breakthrough" infections in 24,706 health workers, mostly caused by Delta.

  • Vaccinated people cleared the virus quicker than unvaccinated people, they said.

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Fully vaccinated people who catch the Delta variant appear to be less infectious than unvaccinated people who have COVID-19, early research suggests.

A Dutch study led by scientists at Rotterdam's Erasmus Medical Center also found that fully vaccinated people who caught the Delta variant got rid of the virus faster than unvaccinated people who caught a less infectious strain.

Particles of the virus in vaccinated people's noses and throats were less likely to be infectious than in unvaccinated people, the study authors said.

This was despite both groups initially carrying a similar level of virus, the researchers said.

The research adds to a growing body of evidence that vaccines help slow the spread of the Delta variant beyond simply stopping infection. In theory, if you're less infectious for less time, you're less likely to pass on the virus. A multi-centre study from Singapore, posted on July 31, reported similar results.

But there's more work to be done before we can draw firm conclusions on how vaccines impact transmission.

Data shows that vaccines work against the Delta variant, and that "breakthrough infections" - i.e., when a vaccinated person gets COVID-19 - are usually mild.

In the Dutch study, researchers examined the COVID-19 lab tests of 161 breakthrough infections in 24,706 fully-vaccinated health workers between April and July. Delta caused most of the infections.

Roughly 85% of the 161 breakthrough infections were symptomatic, but none required hospital admission, the researchers said.

More than half of those in the vaccinated group, which had an average age of 25, had received Moderna's shot, but the authors said findings were similar in people vaccinated with the Pfizer, AstraZeneca, or Johnson & Johnson COVID-19 vaccines.

Delta became dominant after most health workers in the country were vaccinated, so for the control group, researchers tested unvaccinated people who caught a less infectious virus strain between April and December 2020.

Read more: Experts explain why the mRNA tech that revolutionized COVID-19 vaccines could be the answer to incurable diseases, heart attacks, and even snake bites: 'The possibilities are endless'

Vaccinated people can still be infectious

Both the Singapore and Dutch studies measured the number of virus particles, which is considered the best proxy for how infectious somebody is - but it's an imperfect measure that varies from person to person and throughout the course of illness.

Neither study has been scrutinized by other experts in a peer review.

The Dutch scientists cautioned that infectious virus particles were found in 68.6% of breakthrough cases.

The "infectivity of individuals with breakthrough infections should not be neglected," they said.

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