Why is there no herd immunity against coronavirus?

This term became popular at the beginning of the pandemic as the solution to return to normality, but 20 months later it is a "utopia" to achieve it.

During the first months of the pandemic, the term "herd immunity" became popular as the solution to the health crisis that the world has been experiencing for almost two years. This concept is that a disease stops being transmitted when a large part of the population has become immune to it. Thus, everyone is protected against it, even those who are not immune, since the disease stops being transmitted.

This herd immunity can be achieved when a sufficient number of people have had the disease and generated antibodies, or when a large percentage are vaccinated. However, after 20 months of the pandemic, this immunity has not been achieved. According to experts consulted by BBC Mundo, there are several factors that make this unlikely, such as the behavior of vaccines, their uneven distribution and the appearance of new variables.

Factors that prevent herd immunity from being achieved

The Delta variant, twice as transmissible as the original strain, or the Omicron, which seems to escape vaccination with more capacity than the other variants, have complicated the situation. However, vaccination has been shown to be effective in significantly reducing serious illness and death.

"With the vaccines that we have, even if they reduce transmission, the concept of herd immunity does not make sense,” says Salvador Peiró, a doctor specializing in public health and a researcher in pharmacoepidemiology. Therefore, although vaccines save lives, they do not prevent the virus from circulating. This transmission generates an added complication: the possibility of new, more contagious, more virulent or more evasive variants appearing.

Another factor that prevents this immunity is that the protection offered by vaccines lasts for a limited time, between six and nine months, as confirmed by the United States Centers for Disease Control. For this reason, booster doses are already being administered in countries where a large part of the population has the full regimen.

The unequal distribution of vaccines is another element to take into account. While there are countries that far exceed 70%, worldwide only half of the population has received the first dose, the lowest figure being 6.3% in the poorest countries, according to Our World in Data.

"We are not going to win this by vaccinating rich countries every 6 months," says Caroline Colijn, a researcher in pathogen epidemiology and evolution at Simon Fraser University in Vancouver, Canada. " It is extremely important to take a global look and make sure that vaccines are available and used in all parts of the world,” he says.

What does the future hold?

Mauricio Rodríguez, professor at the Faculty of Medicine of the National Autonomous University of Mexico, explains that "herd immunity to COVID is a utopia." "The problem is that COVID is present in all age groups, populations, everywhere and all the time,” he says, making it "practically impossible" to achieve this immunity.

Therefore, you have to get used to living with the virus, something that most experts agree on." It is not a question of eliminating all the cases, what we hope is to have an immune situation with very few serious cases ", details Peiró. "The success of the pandemic is seeing hospitals empty of COVID cases," he concludes.

 

Herd immunity can only be aspired to "in practice"; that is, in certain groups such as people over 60 years of age or people at risk, as Colijn explains. If vaccines are applied in a massive and equitable way, a near-normal life can be achieved, without the need for radical measures.

"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,” he says.