What we know to date about the spread of the delta variant in vaccinated people

The delta variant determines a higher viral load than the others even in those who have completed the cycle, but the contribution of vaccines makes the difference and remains decisive

(photo: Yoav Aziz / Unsplash) As we have known for many months , the main role of vaccines against Covid-19 is to drastically reduce the number of serious cases of the disease, hospitalization or intensive care and deaths caused by the new coronavirus. Alongside this decisive benefit, always confirmed for all variants, the scientific community is questioning how much vaccine formulations are also able to prevent the transmission of the virus from person to person.

And if at the beginning it seemed that the answer could be positive, inducing an optimism even beyond expectations, the question instead seems to be more complex and uncertain when it comes to the delta variant of Sars-Cov-2, which as known has a higher ability to transmit and spread than to the previous ones. So much so that, after the first diffusion in India, it became the predominant variant in many countries, including Italy.

To take stock of the situation, on the basis of the data and scientific evidence available to date, was in the middle of August the journal Nature, which on the basis of 5 studies carried out between the end of spring and summer tried to pull the strings of what we know (and what we do not yet know) about vaccines, variant delta and transmissibility.

Vaccinated people can transmit, but how much?

There has been no doubt about the fact that, despite a complete vaccination course, a person can still spread the virus to others for weeks now. The real question, in fact, is to understand if and to what extent vaccination can reduce the probability of transmission and contagion: in the case of the delta variant, the answer we have is that there may be a certain reduction in transmissibility, even if it is too small to being able to afford to relax the anti-contagion containment measures.

This is in fact a minor effect. According to the most recent study we have, published in pre-print version last August 11 - as well as a report by the US Center for Disease Control and Prevention also this month and a research conducted in Texas during the spring - no significant differences were found in the viral load present in the nasal cavity between vaccinated and unvaccinated people. Or rather, all three studies concluded that even the vaccinated people infected with the delta variant and symptomatic had "a high viral load", certainly enough to infect any close contacts.

More flattering towards vaccines are instead the results of a study still in pre-print (published at the end of July) conducted in Singapore and those of the Reatc-1 program in the United Kingdom, which involved over 100,000 volunteers. In the first case it has been shown that the delta variant viral load remains in the nasal cavity in vaccinated for a shorter period of time than in unvaccinated. In particular, for the first 7 days of infection the viral load was similar between vaccinated and unvaccinated, but only in people with the complete vaccination cycle it then began to decrease significantly and rapidly.

The study British, on the other hand, concluded that the average viral load of vaccinated people is quantitatively lower than unvaccinated. The apparent discrepancy with respect to the other studies, however, was justified by the authors by explaining that in their case also people who were positive for the delta variant but asymptomatic were included in the statistics. Therefore, it can be deduced that the vaccine determines a larger fraction of asymptomatic people, or that in these it leads to a lower viral load than in the case of lack of vaccination coverage.

A different biology, but the mask remains

One of the conclusions drawn by Nature, and in turn put in black and white in various scientific publications that we have already told here on Wired, is that the delta variant of Sars-Cov-2 causes a dynamic of the infection different from the others, as evidenced also by the longer average time of hospitalization and the highest diffusivity.

Waiting to collect more precise and accurate data on the effective viral load that can be generated in vaccinated people, and on how much the vaccinated fraction of the population can contribute to the overall circulation of the delta variant, one thing is certain: the vaccine does not in itself justify the abandonment of contagion prevention measures, from the use of m hand hygiene pad up to distancing.


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