Vaccination cuts risk of infecting others with Covid-19 – new research

The vaccination of one family household member can reduce the risk of Covid-19 infection to others in the house by at least 30 per cent, a review by European infection control experts has said.

The report from the European Centre for Disease Prevention and Control (ECDC) has said there could be reduced transmission because vaccinated individuals have a lower viral load and shorter duration of shedding of the virus to others. It expects overall transmission to fall through vaccinations.

The EU agency said that only one study, as of mid-March, had investigated the effectiveness of a Covid-19 vaccine against transmission to susceptible contacts from vaccinated people.

The study in Scotland of more than 140,000 healthcare workers and 194,000 members of their households found a reported risk reduction for infection of 30 per cent but the ECDC said this was "probably an underestimate and could in reality be as high as 60 per cent".

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In a separate report published in Nature, scientists have found evidence that Covid-19 mRNA vaccines – the Pfizer-BioNTech and Moderna medicines – might reduce infectiousness, helping both to protect the individual who received the jab and prevent that person from infecting others.

In an analysis of positive test results in people infected after inoculation with the mRNA vaccine, scientists found that the viral load in infected individuals was “substantially reduced” for infections occurring 12 to 37 days after the first dose of the vaccine.

“These reduced viral loads hint at a potentially lower infectiousness, further contributing to vaccine effect on vaccine spread,” said the Nature report.

Virus strains

In a review of the risk of transmission from vaccinated people, the ECDC said that the total number of infections is expected to decrease significantly as vaccination coverage increases, “provided that there is a match between the vaccine strains and the circulating virus strains”.

The agency's report said that a preliminary observational study from Israel, where more than half the population has been vaccinated, showed a four-fold reduction of viral load in infections occurring 12 to 28 days after the first dose of the Pfizer-BioNTech vaccine.

“It is not yet clear at this stage whether, and how much, these observed reductions in viral load and duration of shedding would make a vaccinated person less infectious,” the ECDC said.

Reinfection by Covid-19 is “rare”, the agency said, and that a previous infection can provide between 81 per cent and 100 per cent protection from reinfection for a period of between five to seven months, though protection against reinfection is lower in people aged 65 and older.

The ECDC warned that these findings were from studies carried out before the emergence of more concerning coronavirus variants and that earlier immunity may not be as effective or as long-lasting against these variants, particularly the Brazilian and South African strains.

The agency said that there should be decreased transmission overall as natural immunity increases and infections decrease unless variants “induce significant immune escape.”