Currently available vaccines against the new coronavirus are not ‘sterilizing’, that is, they are not designed to prevent replication of the virus in the mucosa of the upper respiratory tract. Thus, a vaccinated person can become infected and, even if they do not develop severe COVID-19, they can transmit the infection to others. That is the theory, and hence the insistence that vaccinated people continue to have to respect the six M: mask, meters of distance, clean hands, fewer contacts, more ventilation and I stay home if I have symptoms.
In the past half a year, studies in numerous countries, involving hundreds of thousands of people in total, have put this theoretical fact to the test in the real world to clarify whether vaccines also stop infection and transmission to others.
Less viral load
In February, the first positive data began to arrive, from the vaccination in Israel – which began on December 20, 2020 -: indeed, there were vaccinated people who continued to be infected, but in them viral load dropped a lot. What is expected is that the fewer viruses a person carries, the less likely they are to transmit them, so what was observed in Israel pointed to “a degree of infectivity [de la persona contagiada] substantially less, ”wrote the authors of the study, published in Nature Medicine.
There is strong evidence that vaccines effectively prevent both symptomatic and asymptomatic infections, thereby reducing transmission among the population.
The same suggested, also in February, a study among British hospital staff that found “strong evidence” that vaccines “effectively prevent infections both symptomatic and asymptomatic ”, and that therefore reduce“ transmission between the population ”, the authors pointed out.
At the beginning of April the US Center for Disease Control (CDC) published the results of a follow-up of some 4,000 vaccinated health workers, who found an efficiency of up to 90% in vaccines when preventing infection.
Positive evidence has also been collected in Spain. A study in Catalonia with more than 100,000 hospital and nursing home workers, and with the residents themselves, found a reduction of between 85% and 96% of the infection.
According to the available evidence, we can affirm that the efficacy of the currently approved vaccines is high and, therefore, the risk of infection after exposure to the virus is very low
A report published at the end of May by the Center for the Coordination of Health Alerts and Emergencies (CCAES) of the Ministry of Health concludes, in light of these studies: “According to the available evidence we can affirm that the efficacy of currently approved vaccines it is high and, therefore, the risk of infection after exposure to the virus is very low ”.
Less virus removal time
Another parameter analyzed is the time that an infected person emits viral particles potentially capable of infecting others. And at this point the results are equally encouraging: a study with the AstraZeneca vaccine found that this time was one week in vaccinated, compared to two weeks in unvaccinated.
For the CCAES report, “there are indications that, in case of infection after vaccination, in addition to the reduction in viral load, the time of elimination of the virus could also be reduced”.
How to confirm whether, in fact, given their lower risk of infection and their low viral load in case of infection, a vaccinated person transmits less COVID-19? The litmus test is to measure indirect protection, that is, infections in the environment of vaccinated people.
In nursing homes for the elderly in Spain, it has been seen that the risk of becoming infected with SARS-CoV-2 was also lower in residents who were not vaccinated, in a very similar way to those vaccinated
In a study carried out in Scotland with some 145,000 health workers and 95,000 partners, it was observed that, two weeks after vaccination, the partners of infected vaccinated workers had a lower risk of infection than the cohabitants of unvaccinated health workers.
In the same vein are the results of the work carried out in residences for the Spanish elderly during the first quarter of 2021. In addition to avoiding “at least 17,000 cases and 3,500 deaths” in the first three months of the vaccination program in Spain, the authors conclude that “vaccination has been very effective in preventing infections, both symptomatic and asymptomatic,” and that it has conferred a high indirect protection: “In nursing homes for the elderly in Spain it has been seen that the risk of becoming infected by SARS-CoV-2 was also lower in residents who were not vaccinated, in a very similar way to those vaccinated.”
However, the caution recommendation remains in force, among other reasons because it is not yet known how long the protection of vaccines lasts.
However, the caution recommendation remains in force, among other reasons because it is not yet known how long the protection of vaccines lasts
As he remembers immunologist José Gómez Rial, of the Hospital Clínico Universitario de Santiago de Compostela and Coordinator of Immunology in the GENVIP Vaccine Research Group, “the operation of vaccines in the real world shows that vaccines seem to reduce contagiousness and stop transmission”, but “also data in real life are showing outbreaks in residences where everyone is vaccinated, asymptomatic or mild symptomatic outbreaks but that indicate that somehow transmission continues among those vaccinated ”.
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