Covid: what has happened to herd immunity 2 years after the start of the pandemic (and how do the new variants affect)



In 2020, during the first months of the pandemic, a concept that sounded hopeful became popular: herd immunity.

The term refers to the fact that when a large part of the population becomes immune to a disease, in this case covid-19, it is unlikely that this disease will continue. transmitting.

In this way, the entire population is protected, even those who they are not immune to disease.

Herd immunity can be achieved after enough people have had the disease and develop antibodies, or that a large percentage of the population has been vaccinated.

After two years of pandemic, however, the complexity SARS-CoV-2 has prevented either of these two scenarios from occurring.

According to experts consulted by BBC Mundo, there are several factors that make it unlikely that a herd immunity against covid-19 will be achieved, and that therefore, efforts should be directed towards maintaining the pandemic under control, not necessarily delete it.

The behavior of vaccines and their uneven distribution, and the appearance of new variants, are some of the main reasons why herd immunity is seen as a meta unrealistic.

There is, however, the possibility of achieving a “Herd immunity in practice”, which could offer certain levels of normality in daily life.

Regardless of the future scenario, experts insist that the vaccination and care measures remain powerful tools to advance the fight against covid-19.

The evolution of the virus

In the two years of the pandemic, SARS-CoV-2 has evolved into variants that, in some cases, have allowed it to be more contagious and be a little more resistant to the effect of vaccines.

The clearest example is the variant delta, which has been shown to be at least two times more transmissible than the original virus.

And as for omicron, early studies suggest that it appears to have a greater ability to escape immunization.

However, vaccines against covid-19 have been shown to be effective in significantly reducing the risk of developing a serious disease or To die.

Vaccinated people, however, they can be contagious and infect others, although to a lesser extent than the unvaccinated.

That’s a first complicating factor.

“With the vaccines that we have, even if they reduce transmission, the concept of herd immunity makes no sense“, Tells BBC Mundo Salvador Peiró, a doctor specializing in public health and a researcher in pharmacoepidemiology at FISABIO, a biomedical research foundation dependent on the Generalitat Valenciana, in Spain.

And he adds that with the transmission rates that are being seen with omicron it probably makes even less sense.

So while vaccines save lives, they can’t avoid keep the virus moving.

And the fact that the virus continues to circulate generates a second complication, and that is that as long as the virus continues to transmit, there is the possibility that they will appear new variants that are more contagious, produce more severe symptoms, or evade the effect of vaccines.

“Any part with large numbers of infections, whether vaccinated or unvaccinated, is a potential source of new variants“, Tells BBC Mundo Caroline Colijn, researcher in epidemiology and evolution of pathogens at Simon Fraser University in Vancouver, Canada.

And Colijn adds another element, and that is that SARS-CoV-2 also infects animals, with which other species can act as a “reservoir” of the virus from which it can be reintroduced into humans.



Waning protection

A third factor is that the immunity that a person achieves after being vaccinated or after recovering from COVID-19 can decrease over time, as indicated by the Centers for Disease Control of the United States.

According to Shabir A. Madhi, dean of the Faculty of Health Sciences and professor of vaccination at the University of the Witwatersrand, South Africa, the immune response after infection or vaccination lasts approximately between six to nine months.

But that may change in the face of the appearance of new variants, so they are already being applied booster dose that increase and strengthen antibodies several months after receiving the vaccine or having suffered from the disease.

Uneven vaccination

A fourth factor is the unequal distribution of vaccines.

In countries like the United States and the United Kingdom, about 70% of the population is already vaccinated with two doses.

But globally, just over half the population has received at least a dose.

And in the lowest-income countries only the 6,3% has received a dose, according to data from Our World in Data.

This increases the risk in many parts of the world that the virus will continue to spread and that new variants will emerge. potentially dangerous.

“We are not going to win this by vaccinating rich countries every 6 months, ”says Colijn.

“It is extremely important to have a global look and make sure vaccines are available and used in all parts of the world. “

In essence, it is useless for a country to be fully protected while other regions of the world remain vulnerable, because the virus knows no borders.


“Herd immunity for covid-19 is a utopia,” says Dr. Mauricio Rodríguez, professor at the Faculty of Medicine of the National Autonomous University of Mexico (UNAM) and spokesman for the commission of said center for issues related to the coronavirus.



According to Rodríguez, herd immunity applies very well to small groups the delimited.

“The problem with covid is that it is present in all age groups, in all populations, everywhere, all the time,” says the expert.

“It is practically impossible achieve herd immunity with the covid ”.

Rodríguez indicates that perhaps what can be aspired to is to achieve immunity in certain groups, such as people over 60 years of age or who have a condition that makes them more vulnerable to the virus.

What is the way out?

According to the experts consulted by BBC Mundo, instead of aspiring to suppress the virus completely, efforts should be aimed at getting used to it. live with the virus, but without posing a serious threat to humanity.

The goal is for it to become a endemic virusIn other words, it continues to circulate among the population but at a manageable level.

Getting to that point is what Peiró calls “having a functional control of the pandemic ”.

“It is not about eliminating all cases, what we hope is to have an immune situation with very few serious cases“Says the expert.

“It is not that people do not get infected, it is let us not fill the hospitals of serious cases ”.

Peiró maintains that the idea is that the covid looks more and more like a cold.

“The success of the pandemic is seeing hospitals empty of covid cases,” he says.

Immunity in practice

Colijn argues that it is unlikely to achieve a herd immunity, but that it is possible to achieve a “Herd immunity in practice”.



This means that if vaccines are applied in a massive and equitable way, almost normal levels of activity can be achieved, without the need for more radical measures such as confinements.

“We have to think about what measures we are willing to maintain forever, perhaps some of those measures are the use of masks, or rapid tests”; says the expert.

“Stopping seeing our friends or family is probably not one of those measures, we can’t do that forever.”

To achieve this “herd immunity in practice” and “functional control of the pandemic”, experts agree that it is important prioritize the most vulnerable groups in vaccination campaigns.

covid test


The idea is to ensure that as many people as possible are protected against serious disease.

“Vaccines have made it possible for us to fight the pandemic almost Without restrictions“, Says Peiró.

“In other circumstances we would all be locked up, with more deaths and more hospitalized, but we have treated delta with all openThis is thanks to vaccines ”.

That scenario, in which the combination of massive and egalitarian vaccination, together with care measures is more like the stage we are entering.

“We are in a transition stage, going from an emergency stage to then being in an endemic stage, which is when the virus will be circulating more regularly, ”says Rodríguez.

“We must not be scared, we have to learn to live with the virus,” concludes the expert.

BBC Mundo

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