People who have been vaccinated against covid-19 and also had a previous infection have a 41 per cent lower risk of passing on the coronavirus if they become infected again, compared with someone who is unvaccinated and has had no past infections.
The finding comes from a study carried out in US prisons that looked at how often people passed on the virus to someone sharing their cell. It was done during the omicron wave.
The research rebuts the idea聽that the only reason to get聽vaccinated against covid-19 is聽to reduce your risk of getting seriously ill, says at Swansea University in the UK, who wasn鈥檛 involved in the work. 鈥淚t鈥檚 a really important finding.鈥�
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Vaccination against covid-19 markedly reduces the chance of dying from the disease or needing hospital treatment. But it is less clear how effective it is at stopping people from catching the virus and from spreading it to others. That is a key question, because people may consider getting vaccinated not just to benefit their own health, but to reduce their chance of passing it on, especially to more vulnerable relatives.
With the earlier coronavirus variants, it seemed that vaccines reduced infections and transmission by significant amounts 鈥� with estimates as high聽as 90 per cent for the Pfizer/BioNTech shot. But when the omicron variant started spreading globally in November 2021, it became apparent that vaccines were less effective at stopping people from catching the virus, although there was still protection against serious illness.
To investigate, at the聽University of California, San Francisco, and his colleagues looked at the records of 35 prisons in California. They analysed transmission within cells with solid doors and walls, from December 2021 to May this year.
During this period, if a prisoner had covid-19 symptoms or tested positive, they were generally moved into isolation, but this didn鈥檛 always happen straight away. From the jail records, the team found nearly 1300 prisoners who tested positive for covid-19 and shared a cell for at least one night with someone who had recently tested negative.
The prisoners who had tested negative spent an average of 2.3聽days with an infected prisoner and more than a quarter of them caught the coronavirus.
The chance of passing on the virus was 21 per cent lower if the聽infected prisoner had had covid-19 before, compared with if聽they had no vaccine and no prior聽infection. It reduced by 24聽per cent if they had had at least聽one dose of vaccine but no previous infection, and by 41 per cent if they had been infected previously and vaccinated.
Transmission risk fell by an additional 12 per cent for each dose of vaccine the infected prisoner had after the first, with some prisoners having had three shots.
鈥淭his study supports the importance of booster doses in reducing population-level transmission,鈥� the team writes.
鈥淲e need to put this message out there that vaccines work not just to reduce serious illness, but to聽reduce transmission,鈥� says Williams. The findings are relevant for the question of whether
covid-19 booster shots should be offered widely, he says. Only certain groups are being offered boosters in the UK, while in the US, the Centers for Disease Control and Prevention recommends them for聽everyone aged 5 and up.
Reference: medRxiv
