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"There is light at the end of the tunnel, but right now the tunnel is long and dark," Maria Van Kerkhove, head of the WHO coronavirus working group, said a few days ago.

"Although we have tools to get through it and more are coming."

The road that leads to the end of the pandemic must still go through some stages, ensuring the production and supply of vaccines is a delicate process, as is their distribution in a context of epidemic rebound.

This path also involves

solving some of the scientific unknowns that still hang about the virus

, such as the duration of immunity or the confirmation that vaccines prevent transmission as well as protect against disease.

The priority of vaccines is to prevent exposed people from developing Covid-19 - especially in its most severe form - and that is why their clinical trials were designed.

Although there is currently insufficient data to definitively answer the question of transmission, scientists are generally optimistic.

"We believe that

immunity from vaccines is going to reduce infections for two reasons,

" explains James Crowe, epidemiologist and director of the Vaccine Center at Vanderbilt University in the United States, "firstly, if the total number of people vaccinated -or previously infected- is very high, we will see a reduction in transmission. Second, because when new infections occur, it is most likely that infected subjects (and already vaccinated) will shed less virus, so they will be less infectious " .

The speeding-up vaccination campaign in Israel is offering some promising signs in this regard, still preliminary.

One of the four major medical organizations in the country, the Clalit Health Fund, recently released the results of an investigation into

the effects of Pfizer's vaccine.

The study has monitored

200,000 people over 60

who received the vaccine in the first days and who later needed a test (due to symptoms or due to contact).

They then compared the results with 200,000 unvaccinated who requested a test in the same period for the same reasons.

The data reflect few differences in the first two weeks after injection but,

from day 14, a significant drop (33%) in infections is observed

in the vaccinated group.

Taking into account that they only received the first dose (the second is necessary to reinforce immunity) and that the body's response is not immediate to the vaccine, scientists believe that this percentage will increase significantly over time and with the second dose .

The results are in line with trials conducted by Pfizer, which suggested that protection appears between one and two weeks after the first injection.

The second prick, three weeks later, would help immune cells to memorize the characteristics of the virus, ensuring long-term protection.

Immune response

"Although it is a small sample, I believe that the study in Israel is a good indication and that it will be confirmed in the future," explains Guy Gorochov, head of the Center for Immunology and Infectious Diseases (CIMI) of the Paris-Sorbonne University and of the Pitié-Salpêtrière Hospital.

"The people who manufacture the highest viral load are the ones with the strongest infections, so by preventing them, the vaccine is going to greatly stop their replication."

The

SARS-CoV-2

produces two types of responses in the immune system, a local, rapid, called IgA antibody and other systemic IgG antibody.

Gorochov and his team describe this mechanism in an article published last December in the journal

Science Translational Medicine

.

"The vaccine is injected intramuscularly and primarily triggers a systemic response through IgG antibodies, which are less present in the upper mucous membranes," explains the immunologist.

Hence that period of 14 days before the infections begin to fall.

"We are currently studying IgA antibodies, which are in the mucosa, because they could play an important role in slowing down the transmission of the virus in the respiratory tract."

Vaccine Policy

Confirming that immunity prevents infections is essential.

First of all, because the idea that group immunity means the end of the pandemic - preventing the circulation of the virus - rests on this principle.

Furthermore, it is information that can guide vaccine policy;

If it is a true barrier against the virus, it

may be a priority to immunize potential super pollutants

, those who interact with more people.

Previous studies carried out by Moderna had already provided some guidance, first in animal tests, then in phase 3 trials. The American laboratory carried out PCR tests on 28,000 volunteers before the two injections of their vaccine: of the 52 who gave positive four weeks later only 14 received the vaccine, 38 had received a placebo.

"Not enough to be conclusive, but it suggests a degree of protection," Moderna's team said at the time in an article in the

New England Journal of Medicine

.

But, to eliminate any doubt, more tests are still necessary.

One possibility is to

obtain nasal samples from people immunized with the vaccines already licensed

.

"But investigations of this type are more complicated, because it is more complex to observe the responses in healthy individuals, for ethical reasons," explains Gorochov.

"It requires many more authorizations, even though it is not invasive exams."

A trial planned by the French platform Covireivac with Moderna's vaccine aims to move in this direction, similar to the Israeli study.

Another alternative solution

would be to carry out clinical tests exposing vaccinated people -

young and without prior risks - to study the effect of the virus on their bodies, particularly on the nose, a possibility that is being explored in the UK.

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