Governments and officials hope that vaccines against COVID-19 will be able to build “herd immunity”, and some estimate that immunizing only two-thirds of the population could stop the pandemic and help protect entire communities or countries.
However, this concept leads to warnings that it is highly desirable in terms of the benefits of vaccines. Some experts say such expectations are wrong.
To begin with, a number of factors, some of which are unknown, need to be considered in order to understand what is needed to achieve herd immunity through COVID-19 vaccines.
What is the rate of spread of the virus that causes the COVID-19 disease? Will the first available vaccines be able to stop the transmission of the virus or will they just help people not get sick? How many people in a population will be vaccinated? Will vaccines provide equal protection for all?
“Herd immunity is sometimes misunderstood as individual protection,” said Josep Jansa, an expert on emergency preparedness and response at the Stockholm-based European Center for Disease Prevention and Control (ECDC).
“It’s inappropriate to think, ‘I’m not going to get infected myself because I have herd immunity.’ Herd immunity is about protecting the community, not how well the individual is protected,” he said.
The center uses a calculated 67 percent herd immunity threshold for its models, although German Chancellor Angela Merkel said this month that restrictions on COVID-19 in Germany could be lifted if between 60 and 70 percent of the population is immune. , either by vaccines for COVID-19 or after infection.
Experts from the World Health Organization (WHO) also point to vaccine coverage of 65-70 percent of the population as a way to achieve mass immunity through vaccination.
“The point of herd immunity is to protect the vulnerable,” said Eleanor Riley, a professor of immunology and infectious diseases at the University of Edinburgh. “The point of this is, if we say, 98 percent of the population has been vaccinated, they will remain so little infected in a community that the other 2 percent will be protected. That is the goal,” she added.
The reproduction index is crucial
The virus reproduction index plays a major role in the calculations of this COVID-19 concept. It is an average indicator of how many other people an infected person transmits the pathogen under normal conditions without restrictive measures.
Assuming that the vaccines are fully effective, the threshold percentages of herd immunity for infectious diseases are calculated by a special formula: 1 is divided by the reproduction index, the result is subtracted from 1 and the number obtained is multiplied by 100.
For example, herd immunity to a highly contagious measles disease with a calculated reproductive index of 12 or higher will only occur if 92 percent or more in a given group are immunized. For seasonal flu, the value of the index is 1.3, and the threshold would be only 23 percent.
“The problem is that so far we don’t know exactly how quickly the virus spreads without precautions and during normal travel and social activities like we had a year ago,” said Winfred Pickle, a professor of immunology at the Medical University of Vienna.
With so many countries living in conditions far from normal, the assumption should be that the value of the COVID-19 index will be “closer to 4 than to 2”, he said, given that even with the measures for partial or complete quarantine the value of the index is close to 1.5.
In addition, anything less than 100 percent vaccine efficacy, such as 90 percent or other, as suggested by early data for Pfizer-BioNTech and Moderna vaccines against COVID-19, would require sufficient increase in the percentage of coverage to reach the herd immunity threshold.
Amesh Adalca, a scientist with the Johns Hopkins Center for Health Security, said a good goal for immunity in the United States would be to vaccinate more than 70 percent of the population. However, he agreed that a larger share could be needed if vaccines were less effective.
Will the transfer to the “herd” stop?
Experts say another important factor is whether the COVID-19 vaccines a government chooses to use can stop the virus from being transmitted.
Current data show that the first vaccines against COVID-19 to be released will at least prevent the development of the disease. However, it cannot be ruled out that people will continue to become infected with the virus and pass it on unnoticed to others.
“Although protection against the virus is valuable to the individual, it will not prevent the circulation of the virus and the risk of disease to unvaccinated (humans),” said Penny Ward, a visiting professor of pharmaceutical medicine at King’s College London.
Bodo Plachter, lecturer and deputy director of the training hospital at the Institute of Virology at the University of Mainz, Germany, said that respiratory infections, in particular, are difficult to completely prevent with vaccines, although vaccinations will help reduce circulation. virus.
“It may turn out that vaccinated people will spread the virus less,” he said, adding: “But it would be a mistake to think that only vaccination can suppress a pandemic.”
Should we focus on protecting the vulnerable?
The Edinburgh lecturer Riley said that this suggests that for now, pursuing the goal of herd immunity through vaccination against COVID-19 is useless.
A better approach, she said, could be to “do the exact opposite of herd immunity,” and use the first limited doses of the vaccine to protect those most in need, without worrying about healthier members. the “herd” that can live relatively well with the virus.
“Let’s forget about protecting the masses to protect the vulnerable,” she said, urging, “Let’s protect the vulnerable directly.”