Coronavirus vaccines are extremely effective at preventing the spread of the Alpha variant of the disease but the impact may be less on the more infectious Delta variant, according to researchers at Dutch public health institute RIVM.

The researchers studied how often people who have been fully vaccinated infected others in their household between February and May, when the Alpha variant was dominant in the Netherlands.

The study showed that people living in the same household as people who were fully vaccinated, but picked up coronavirus, were 71% less likely to be infected than household members of unvaccinated people.

Among people who were infected after partial vaccination (at least two weeks after the first jab with the Pfizer, AstraZeneca or Moderna vaccine), 21% fewer household members became infected.

The research results also show that if someone in the household tested positive, fully vaccinated household members were 75% less likely to become infected than unvaccinated household members.

This is slightly lower than post-vaccination protection in studies of the whole population, probably because household members of infected individuals are exposed to high levels of virus particles, the RIVM said.

Delta

‘Things may look different with the Delta variant,’ chief researcher Brechje de Gier told broadcaster NOS. ‘But we cannot draw firm conclusions based on this data. But this research does show that vaccines work very well in stopping the virus being passed on, and it is the first time this has been so clearly proven.’

Over the next few months, the RIVM will be monitoring whether the vaccines also offer protection against transmission of the Delta variant.

The full results of the survey will be published in Eurosurveillance, an open-access medical journal on infectious diseases, later on Thursday.

The RIVM said on Wednesday that 85% of people over the age of 18 have now had at least one jab and 66% have been fully vaccinated. Youngsters aged 12 to 17 are also being urged to get fully vaccinated.