One of the biggest unknowns that have surrounded COVID-19 vaccines since their commercialization has been their ability to prevent people vaccinated and infected by SARS-CoV-2 from infecting other individuals. Most of the clinical trials carried out did not investigate this question because it made it difficult to carry out the studies and because it was not a priority objective at all. The primary focus of clinical trials is to understand the safety and efficacy of these drugs in preventing severe COVID-19 and deaths from novel coronavirus infection.
Group immunity is illusory and we still have 10.5 million people to vaccinate
Despite this remarkable initial ignorance about the ability of vaccines to stop infections, this issue has always been important due to its implication in the control of the pandemic. The greater the effect of vaccines to prevent the transmission of the coronavirus from vaccinated people, the easier it will be to control the virus, by limiting epidemic waves. However, knowing this key detail is not easy for different reasons.
Why is it not easy to know if a vaccinated person can infect?
In the first place, because detecting coronavirus infection in vaccinated people is more complicated, since in these it usually presents asymptomatic or with very mild symptoms. So, from an epidemiological point of view, identifying these contagious people has added difficulties and they are more likely to escape the “radars” when tracking contagions. To identify all the infected people (even if they do not have symptoms) in a group, it is necessary to carry out PCR tests on all of them, which implies the use of considerable resources and personnel.
Another factor that complicates this issue is that it is not at all easy to know how contagious an infected person is (vaccinated or not) from laboratory tests. The amount of viral particles that is detected from a sample obtained from the patient’s fluids, also known as the viral load, is not a good indicator of the contagiousness of a person. There are also many other factors that influence the transmission of the coronavirus: the ability of the person to release droplets / aerosols with viral particles to the outside, the variant involved, the infective capacity of the viruses in respiratory fluids, the duration of the infection … .
What do we know about whether vaccines limit infections?
At this time, more than eight months after the first COVID-19 vaccines were marketed, we already have more information about their usefulness in blocking contagion from inoculated ones. Multiple epidemiological studies indicate that fully vaccinated people have a lower risk of SARS-CoV-2 infection, and not just high protection against COVID-19. The Centers for Disease Control and Prevention has compiled the results of various studies that analyze the occurrence of SARS-CoV-2 infections between vaccinated and unvaccinated. The figures range between 77% and 99% reduction of infections among those immunized.
An article published in The New England Journal of Medicine shows that commercialized messenger RNA vaccines were very effective in the real world in preventing SARS-CoV-2 infection among essential workers in the United States. Of the 204 participants in whom the coronavirus was detected, only 5 were fully vaccinated and 11 partially vaccinated. With respect to the rest of the infected people, 156 were unvaccinated and in 32 their vaccination status was unknown. In addition, these vaccines decreased the viral load, the risk of feverish symptoms and the duration of the illness among those who did become infected despite having been vaccinated.
Another study, conducted in the Netherlands, observed very similar results between close and household contacts: vaccines against COVID-19 not only offered some protection against infection, but also reduced the risk of contagion between close contacts after completing the vaccination regimen. The authors note that these results highlight the importance of vaccinating close contacts of vulnerable people, thereby reducing the risk of infection. In Catalonia, a study carried out among the elderly and nursing home staff and health workers, it found that vaccination was associated with an 80-91% reduction in the risk of SARS-CoV-2 infection.
The rise of the delta variant in England between May and early July 2021 was a litmus test for vaccines. Again, the researchers observed that people with the full vaccination schedule had a lower risk of coronavirus infection than unvaccinated or partially vaccinated people.
Antonio Gutiérrez, pharmacist and epidemiologist at the Virgen de Valme University Hospital, cites studies that suggest that “people who are fully vaccinated are less contagious than those who are not vaccinated, and also, according to data from ago few days on preprint still (preliminary articles not yet peer-reviewed), the vaccinated would eliminate the virus from their body much faster than the unvaccinated, a fact more than relevant given the prevalence of the delta variant, as it is much more transmissible than previous variants ” .
Why don’t vaccines totally prevent contagion?
Although there were no studies quantifying this phenomenon at the beginning, it was already anticipated that the vaccines currently marketed would not guarantee the total blockage of the infection or prevent the transmission of the coronavirus from vaccinated people. It is therefore no surprise that we see people vaccinated and infected. It is not a problem of vaccines, but of unrealistic expectations about their benefits that have been generated and transmitted due to lack of information.
As these drugs are designed, they cannot 100% prevent the passage of SARS-CoV-2 into the human body (they do not offer sterilizing immunity), since the main immune protection they induce takes place inside the body and not in the barriers that defend it (oral and respiratory mucosa, saliva …). In other words, current vaccines are highly effective in stimulating our immune system to respond strongly to the coronavirus once it is inside the body (avoiding serious diseases and death), but they do not prevent In many cases, this pathogen crosses the barriers and enters, thus causing an infection.
In any case, as Antonio Gutiérrez explains to eldiario.es, current vaccines do have a certain capacity to prevent the coronavirus from entering our body: “We already have data that even current (intramuscular) vaccines generate IgA-type antibodies , in saliva and oral mucosa, more focused on defending ourselves in the same ‘gateway’ of the virus “. It is an incomplete protection in this area, but sufficient to reduce the risk of infection among those vaccinated and, therefore, also reduce the transmission of the coronavirus.