Vaccines protect against further spread of the virus in 2 ways: they reduce the risk of infection and make the vaccinated person less contagious if they do become infected.
This is apparent from a study (link is external) by Sciensano, the Belgian institute for health, which was published in the international professional journal Vaccine. The findings date from the period before the delta variant became dominant in Belgium.
Vaccinated persons less susceptible to infection after high-risk contact
In Belgium, for the period from January 25 to June 24, 2021, no distinction was made between people who had already been fully vaccinated and people who had not yet received a vaccine: after a high-risk contact, quarantine and two PCR tests followed for everyone.
Sciensano, the Belgian health institute, is now using those test results and contact tracing data to find out how well vaccination protects against coronavirus infection.
The researchers compared the risk of a positive test of 8,000 vaccinated high-risk contacts with that of 282,000 unvaccinated high-risk contacts.
This shows that complete vaccination with an mRNA vaccine (Pfizer or Moderna) reduced the chance of infection by 74 to 85%. Protection was clearly better after full vaccination than after only 1 dose of an mRNA vaccine. This study takes into account all positive tests, so also in people who showed no symptoms.
The degree of protection against the coronavirus is therefore slightly lower in this study than in studies into protection against (serious) symptomatic disease.
Fully vaccinated people with a positive COVID-19 test passed on the virus less often
If someone still gets infected after full vaccination, is that person just as contagious as an unvaccinated person? The researchers looked at infections between 990 people with an infection after full vaccination (so-called ‘index cases’) and their unvaccinated high-risk contacts.
The vaccinated index cases transmitted the virus 52 to 62% less often than unvaccinated index cases.
Both types of protection can also co-exist: if both the index case and its high-risk contact are fully vaccinated, there are about 90% fewer infections than contacts between unvaccinated individuals.
Still be careful
Despite these positive results, it is important to keep in mind some caveats.
This study was conducted during the period when the alpha variant was dominant in Belgium. There is a possibility that the protection against the delta variant is less effective. However, the researchers do not yet have concrete information about this.
In addition, this study does not include contact tracing data in, for example, residential care centres, which runs through a separate system. It is possible that protection in the elderly is lower than in the active population. Here, too, the researchers do not have the necessary information.
Finally, the majority of test results during the study period were from unvaccinated individuals.
It was not possible to make reliable statements about the protection provided by the AstraZeneca and Janssen vaccines, with which only a limited number of people had been fully immunized at the end of June.
Sciensano emphasizes that infections between vaccinated people can still occur and that it is therefore important that everyone follows the current rules.
Since the end of June, the quarantine rules have been less strict for vaccinated high-risk contacts. Photo AZ / jp / N
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