ECDC report examines SARS-CoV-2 transmission risk from vaccinated/previously infected individuals
In the Technical Report 'Risk for SARS-CoV-2 transmission from newly infected individuals with documented previous infection or vaccination', published today, ECDC examines the evidence on the extent previous SARS-CoV-2 infection or COVID-19 vaccination prevents onward transmission from infected individuals to susceptible contacts.
“It is very encouraging to see that SARS-CoV-2 reinfections are quite rare. We also welcome the positive early indications that the risk of onward transmission seems to be reduced in those who are vaccinated. Although the effect of new variants of concern on transmission patterns needs to be closely monitored, we still expect that the total number of infections will significantly decrease as vaccination coverage increases” said Andrea Ammon, ECDC Director.
Concerning transmission from previously infected individuals, the following key points were concluded:
- No studies directly measured SARS-CoV-2 transmission from reinfected individuals to their contacts. However, there was evidence showing that reinfections are rare. Studies that have followed people for 5 to 7 months after recovery from a SARS-CoV-2 infection have estimated that the protective effect of previous SARS-CoV-2 infection is very high (81% – 100%) during that period. It should be noted however, that many of these studies were carried out before the emergence of SARS-CoV-2 variants of concern and there is weak evidence that immunity induced against previously circulating SARS-CoV-2 strains may not have the same potency against variants of concern, in particular B.1.351 and P.1.
Among vaccinated individuals, ECDC’s review showed the following:
- Evidence of the impact of vaccination on the risk of transmission is available from just one study, which suggests that vaccination of a household member reduces the risk of infection in susceptible household members by at least 30%.
- There is evidence that vaccination significantly reduces symptomatic or asymptomatic infection in vaccinated individuals, although the vaccine efficacy varies by vaccine product and target group.
- There is also some evidence of lower viral load and shorter duration of shedding in vaccinated individuals as compared to unvaccinated individuals, which could translate into reduced transmission.
- Many of the studies on vaccine effectiveness were carried out before the emergence of SARS-CoV-2 variants of concern, and there is some evidence that vaccine efficacy could be decreased for some of the variants, in particular for B.1.351 and possibly also P.1.
Follow up of cohorts with previous SARS-CoV-2 infection and vaccination is needed to better assess the magnitude and duration of protection from reinfection and symptomatic disease, as well as the effect of protection against transmission.
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