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Vaccines, a firewall against covid-19

This article was published on
July 30, 2021

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If there is one good thing to be said about this pandemic, it is that it allows us to admire the advance of knowledge in real time. When vaccination began, it was only known that vaccines prevent severe disease; several months - and numerous studies - later, it can be said that they also significantly reduce infection and transmission.

If there is one good thing to be said about this pandemic, it is that it allows us to admire the advance of knowledge in real time. When vaccination began, it was only known that vaccines prevent severe disease; several months - and numerous studies - later, it can be said that they also significantly reduce infection and transmission.

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Information and context on vaccines. Article written by the science journalists of the SINC agency with the analysis and review of expert sources.

The vaccines currently available against the new coronavirusare not ‘sterilising’, i.e. they are not designed to prevent virus replication in the mucosa of the upper respiratory tract. Thus, a vaccinated person can become infected and, even if he or she does not develop severe covid-19, can transmit the infection to others. That is the theory, hence the insistence that vaccinated people must still respect the six basic rules: mask, metres away, clean hands, less contact, more ventilation and staying home if I have symptoms.

Over the past half 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 curb infection and transmission to others.

Reduced viral load

February saw the arrival of the first positive data of vaccination in Israel - which began on 20 December 2020: vaccinated people were indeed still being infected, but their viral load was much reduced. It is to be expected that the fewer viruses a person carries, the less likely they are to transmit them, so what was observed in Israel pointed to “a substantially lower degree of infectivity [of the infected person],” wrote the authors of the study, published in Nature Medicine.

The same was suggested, also in February, by a study of British hospital staff that found “strong evidence” that vaccines “effectively prevent both symptomatic and asymptomatic infections”, thereby reducing “transmission in the population”, the authors noted.

Fewer infections

In early April the US Centers for Disease Control (CDC) published the results of a follow-up of some 4,000 vaccinated healthcare workers, which found the vaccines to be up to 90% effective in preventing infection. Positive evidence has also been collected in Spain. Work in Catalonia with more than 100,000 hospital and nursing home workers, and with the actual residents, found a reduction of between 85% and 96% in infection.

A report published at the end of May by the Ministry of Health’s Centre for the Coordination of Alerts and Health Emergencies in Spain (CCAES) concludes, in the light of these studies: “From the available evidence, we can state that the efficacy of the currently approved vaccines is high and, therefore, the risk of infection after exposure to the virus is very low.”

Shorter virus clearance time

Another parameter analysed is the time an infected person emits viral particles, which are potentially infectious for others, into the environment. And here the results are equally encouraging: a study with AstraZeneca's vaccine found that this time was of one week among vaccinated people, compared to two weeks among unvaccinated people.

For the CCAES report, “there are indications that, in case of infection after vaccination, in addition to the reduction in viral load, thetime for viral shedding may also be reduced.”

Indirect protection

How to confirm whether, given its lower risk of infection and low viral load in case of infection, a vaccinated person transmits less covid-19? The acid test is to measure indirect protection, i.e. infections in the environment of vaccinated people.

A study made in Scotland with some 145,000 healthcare workers and 95,000 cohabitants found that, two weeks after vaccination, the infected cohabitants of vaccinated workers had a lower risk of infection than the cohabitants of unvaccinated healthcare workers.

The results of work carried out in Spanish nursing homes during the first quarter of 2021 are along the same lines. In addition to preventing “at least 17,000 cases and 3,500 deaths” in the first three months of the vaccination programme in Spain, the authors conclude that “vaccination has been very effective in preventing infections, both symptomatic and asymptomatic”, and that it has conferred high indirect protection: “In elderly care homes in Spain, the risk of becoming infected with SARS-CoV-2 was also lower among residents who were not vaccinated, in much the same way as in those who were vaccinated.”

Remaining cautious

The recommendation for caution remains, however, not least because it is not yet known how long the protection of the vaccines lasts.

As immunologist José Gómez Rial, from the University Hospital Clinic of Santiago de Compostela, also the Coordinator of Immunology at the GENVIP Vaccine ResearchGroup, recalls, “the functioning of vaccines in the real world shows that vaccines seem to reduce contagiousness and curb transmission,” but “real-lifedata are also showing outbreaks in residences where everyone is vaccinated, asymptomatic or mild symptomatic outbreaks but which indicate that somehow there is still transmission among those vaccinated.”

 

This article is also available in Spanish.

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