Group immunity is not a guarantee against Covid-19


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Spain has been one of the countries most affected by the Covid-19 pandemic and was one of the first to adopt extreme measures of confinement when the coronavirus swept the nation between March and April 2020. However, since that first scenario, Spain has It has presented better data than many of its European neighbors, even despite a sharp increase in Covid cases during the winter of 2020-21, unlike other countries, such as the United Kingdom, Italy, France and Germany, which have returned. to harsher confinements.

One of the causes of this scenario has been, according to an article published in
«The Lancet»
, the high vaccination rate. Currently it is estimated that more than 85% of the population of Spain has received the complete guideline and we are beginning to talk about something that until a few weeks ago seemed unlikely: herd or group immunity.

“We still do not know the exact proportion of the population that needs to be immune to achieve herd immunity to SARS-CoV-2, as we need to better understand the duration and protection of transmission generated by both vaccination and Covid- 19 previous infection ”, he explains in
«The Lancet»
Jesús Rodríguez Baño, head of infectious diseases of the
Virgen de la Macarena Hospital in Seville
, Spain.

Group immunity is defined as the resistance of a group to the entry and spread of an infectious agent on the basis that there is a high proportion of people who are immune to the infection.

So, has Spain achieved the long-awaited group immunity? “If we stick to the definition of group immunity, the answer is no,” says Ángela Domínguez, coordinator of the Working Group on Vaccination of the Spanish Epidemiology Society.

And, says Dr. Domínguez, “right now in Spain there is a 14-day incidence of more than 50 cases per 100,000. That is, – he warns – there is circulation of the virus among the population ». And, he adds, although we are at medium / low risk, “there is still income at the plant and at the ICU. There is little transmission, but there is.

However, Jesús Rodríguez Baño points out in
«The Lancet»
, «The situation in Spain may give some clues: after leaving behind most of the control measures in the population, the contagion rate (and especially the rate of hospital admissions) has been decreasing, and this is the opposite of what happened in the previous waves. The only plausible explanation is the very high vaccination rate in the country.

In this message, Ángela Domínguez agrees: «We are at low risk, probably thanks to the high vaccination coverage, almost 88% of the target population. And even so, there are still admissions in the plant and in the ICU. There is no doubt that vaccination is one of the keys that we have a low incidence, but it is not the new ‘normal’, which would be a figure lower than 25 per 100,000 “, he emphasizes.

This epidemiologist explains that group immunity largely depends on the transmission capacity of the virus (RO factor).

At the beginning of the pandemic, he points out “the RO factor of the coronavirus was 2 or 3 (1 person infects 2 or 3 individuals), but it has been seen that it can be 6 or 8. Thus, if, for example, we apply an R0 out of 6 (1 person infects 6 individuals), we need 83% of the population to be immunized.

Since the vaccine is not 100% effective, but 80 or 85% effective, we would need 97% of the immunized population, either through the vaccine or by natural infection. That is to say, that 97% would have to take away the percentage of the population that has passed the Covid-19 and that is immune.

But, he cautions. “We still don’t know how long that immunity lasts. We do not know, neither for those who have passed the infection nor for vaccinated people.

In fact, a study published this week in
«Science»
that has analyzed the possibility of infection and the capacity of protection against Covid of the vaccines of Moderna, Pfizer, Janssen among 780,225 people showed that the protection against Covid-19 decreases for all types of vaccines: from 87.9% in February to 48.1% in October 2021.

The problem, Domínguez continues, is that group immunity models are made on the basis that immunity is «for life, as with smallpox or measles, or that the virus does not change. And in this case it does not seem that these two situations exist».

The conclusion, he stresses, “is that there is no magic number. We will always have to consider these two variables: OR and the effectiveness of the vaccine.

We still don’t know how long that immunity lasts. We do not know, neither for those who have passed the infection nor for vaccinated people

«Group immunity is something that can help, but it is not a guarantee», Emphasizes Dr. Domínguez, who is committed to« changing the chip. Rather than talking about group immunity, we must continue with vaccination and control measures. In this way we will make the transmission as low as possible and we will be able to assess whether a third dose is necessary or not ”.

Rodríguez Bath is hopeful that winter 2021-22 in Spain will be something much more normal than what it experienced last year. However, it points in
«The Lancet»
that ‘we must be cautious in our predictions. The third booster dose is already being administered in high-risk groups here in Spain, and we have yet to see if a third dose (or an annual dose) is necessary for everyone. ‘

Meanwhile, in the rest of Europe the picture is confusing. The United Kingdom, which opened businesses and removed all restrictions in July 2021, has seen daily rates that have reached 50,000 daily cases and has a cumulative incidence of 14 out of 753 per 100,000 inhabitants, around 20 times that of Spain.

All this has led the British medical community to request that some control measures be reintroduced, such as masks indoors and return to teleworking when possible.

The government of Denmark lifted all restrictions in early September 2021 and decided that the time had come to accept that you have to live with Covid-19 like any other endemic disease. To date, this strategy appears to have been very effective, although daily new infections across the country have doubled.

At the end of September, Norway also announced a similar strategy. So far, the country has not experienced a sharp increase in cases. Denmark (76%) and Norway (69%) have lower proportions of the vaccinated population than Spain.

I wish it were as simple as achieving group immunity anxiety and that the problem would end and that’s it

With winter approaching Europe, the next few weeks and months will be crucial in discovering how successful each country’s individual strategies have been in controlling the coronavirus.

At the moment, Spain, which started out being highly criticized, is the country with the best data. But, as the epidemiologist points out, “vaccination must be continued in groups where it is still low, such as people between 20 and 39. And also with non-pharmacological measures: confinement, quarantine of contacts, masks, social distancing , etc.”.

Because, he repeats, «the fact that the incidence has decreased does not mean that group immunity has been achieved». And remember that pandemic control is not just about vaccination coverage.

“I wish it were as simple as achieving the long-awaited group immunity and for the problem to end and that’s it,” concludes the expert.

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