Divulgação, Notícias

Herd immunity for COVID-19 is a dangerous mirage

EIt is estimated that at least 60% of the world population needs to be immunized before the concept of herd immunity shows any effect


Following the current rate of vaccination, the number of deaths may more than double in the second half of Brazil

The World Health Organization (WHO) reinforced at the beginning of the year that the world will not achieve the so-called herd immunity (collective or group) against COVID-19 in 2021, despite the fact that several countries are already applying vaccines against the coronavirus. We are not going to achieve any level of collective immunity in 2021 because the vaccine application process takes time, said WHO chief scientist Soumya Swaminathan at a virtual press conference in Geneva. It is estimated that at least 60% of the world population needs to be immunized before the concept of herd immunity shows any effect, but that figure is still inaccurate and may be even higher. Some experts speak of an 80% threshold.

The professor at the Laboratory of Health Intelligence (LIS) at the Ribeirão Preto School of Medicine at the University of São Paulo (FMRP-USP) Domingos Alves, agrees and explains that collective immunity implies a barrier of immunized people that impedes the virus transmission cycle. Unlike what was said during the entire pandemic, in which collective immunity was contested by contagion, this concept applies to vaccination campaigns. For him, using the idea of collective immunity as an alternative to controlling the pandemic is absurd. Complementing the justification, the professor cites an excerpt from the technical note of the Congregation of the School of Public Health of USP which reiterates that for scientific, ethical and legal reasons, collective immunity due to contagion cannot be accepted by the State and by Brazilian society as a strategy to respond to epidemics, even less in the face of a pandemic and a disease whose long-term effects are still ignored. In 2020, WHO Director-General Tedros Adhanom Ghebreyesus also stressed that collective immunity was never used in the history of public health as a strategy to respond to an epidemic, let alone a pandemic. It is problematic from a scientific and ethical point of view.

For Professor Alves, collective immunity is possible and must be achieved through effective vaccination of the population. According to him, immunization by means of the vaccine is urgent and essential. “The minimum percentage of the population that needs to be immunized to achieve immunity varies depending on the disease. In the case of COVID-19, if the effectiveness of the vaccine is somewhere between 90% and 95%, as the laboratories have said, immunizing about 70% of the population may be sufficient. If it is less than 70%, it will be necessary to vaccinate between 85% and 90% of people to generate collective immunity”, he adds. All countries that are in a better vaccination process than Brazil have adopted measures to break the chain of transmission of the virus.

Waiting for herd immunity to achieve normality is not an option

Until March 31, just over 17 million people (8.16% of the population) were vaccinated with the first dose and just over 5 million (2.38%) with the second, that is, the daily rate of vaccination is still quite low (8.51 per 100 people). “At the current rate (number of people vaccinated per 100 people), in this more optimistic scenario of vaccinating 70% of the population, we should not achieve herd immunity before 2023. And to vaccinate 70% of the population by the end of the year, we need to increase the speed 10 to 11 times, warns Professor Domingos Alves, noting that Brazil is experiencing the most acute moment of the pandemic. April will be the worst month of the pandemic: in the beginning we must exceed 100 thousand cases per day in the moving average and we must reach 3500 to 4000 daily deaths in the first days of the month”, already warned the expert. For him, in face of the slow vaccination and the scenario of evolution of the epidemic, we must do like several countries with a vaccination process earlier than ours and adopt restrictive measures (national lockdown of at least 15 days) to contain the spread of the virus. “Waiting for herd immunity to reach normality is not an option”, emphasizes Professor Alves

The more the virus circulates, the greater the chances of new mutations that may cause reinfections to appear, which undermines the possibility of herd immunity. Every epidemiological model has a flow composed of at least three groups: susceptible to the virus, infected and recovered. With the possibility of reinfection, part of the recovered group is again susceptible, that is, it returns to square one, restarting the entire cycle. So far, the new variants (mutations) that have appeared in Brazil, such as in Manaus, have already had a very high prevalence in the country. A recent study carried out in partnership with the USP Institute of Tropical Medicine found that this variant represents 64% of those infected in the city of São Paulo. And everything points that the higher prevalence of these new strains in Brazil is associated with the lack of control (more effective measures, such as sanitary barriers) of the pandemic.

“When we talk about herd immunity from the vaccination process, the indications presented by the laboratories responsible for the vaccines are that they are effective for these new variants, that is, the herd immunity should not be affected by the strains detected so far. However, considering the speed of vaccination and the current scenario of the epidemic in the country, we must have to deal with the virus for several months and maybe years”, observes the professor. Another aspect that should be remembered in the virology of Coronavirus, since its discovery in the 80s, is that there is consolidated evidence that the immune memory induced by the virus is not long lasting (both in the case of those who developed the disease as for the asymptomatic cases). In this sense, even with a complete vaccination coverage of the population, there is the possibility of other outbreaks in the coming years.

New peak of the disease in October

If vaccination is not accelerated, Brazil could accumulate 640 thousand deaths until October, when the disease would reach a new peak. This is what some experts say. Asked when the disease should reach a new peak, Professor Alves is categorical in stating that any mathematical model that ventures to answer this is doomed to error. “Looking at my previous estimates, it is very likely that we will reach 500 thousand deaths by mid-May. Thus, assuming that the peak prediction for October is correct and adding my considerations about the speed of vaccination and the lack of more effective measures to curb the spread of the virus, the number of deaths by October would be inconceivable and indecent”, laments the researcher.

All the projections made by Professor Alves and several Brazilian scientists do not consider the deaths due to the overcrowding of beds and the lack of inputs. “We are already experiencing the greatest health calamity in the history of the country and in order to save lives (many lives) we urgently need to speed up vaccination and print more restrictive measures in Brazilian municipalities. The whole scenario of prediction presented here is conservative in view of the seriousness of what is happening in Brazil at the moment”, concludes the professor.

Postponing the second dose of the vaccine as some countries are already doing may also cause a delay in collective immunity in the long term, according to a study published in Science, although it helps to reduce the number of cases in a short period, according to the article Epidemiological and evolutionary considerations of SARS-CoV-2 vaccine dosing regimens. The study also brings important results associated with imperfect immune responses with regard to the potential for viral immune escape. The aforementioned hypothesis predicts that, due to people with partial immunity, an evolution of the virus may occur.

Even with vaccination efforts in full swing, the theoretical limit for defeating COVID-19 appears to be out of reach. Read the article that outlines the five reasons why COVID herd immunity is probably impossible.