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Governments and officials have expressed hope that COVID-19 vaccines could lead to “herd immunity,” with some estimating that the immunity of just two-thirds of the population could stop the pandemic and help protect entire communities, or states.
But the idea is accompanied by warnings and high demands on which vaccines they can prevent. Some experts say those expectations are wrong.
First, what is needed to achieve herd immunity with COVID-19 vaccines involves a variety of factors, many of which are unknown.
What is the rate of spread of COVID-19 caused by the virus? Will the first vaccines being developed be able to stop the transmission of the virus or will they just help people to stop getting sick? How many people in a population will receive a vaccine? Will vaccines offer the same protection to everyone?
“Herd immunity is sometimes misinterpreted as personal protection,” said Josep Jansa, an emergency preparedness and response specialist at the European Center for Disease Control (ECDC), based in Stockholm. .
“It is not correct to think ‘it will not affect me because there is herd immunity.’ “Herd immunity is about protecting the community, not about how you protect an individual.”
The ECDC uses an estimated lower herd immunity threshold of 67% for its models, and Chancellor Angela Merkel said this month that restrictions on COVID-19 could only be lifted if 60% to 70% of the population were immune. either through a COVID-19 vaccine or through an infection.
Experts from the World Health Organization also said that a vaccination coverage rate of 65% -70% is a way to achieve the immunity of the population through vaccination.
“The idea of herd immunity is to protect vulnerable groups,” said Elinor Riley, professor of immunology and infectious diseases at the University of Edinburgh. “And the idea behind this is that if, say, 98% of the population is vaccinated, there will be so little virus in the community that 2% will be protected. That is the point”.
Reproduction rate is a critical factor
A fundamental aspect of the public health calculations for this COVID-19 idea is the replication rate, or R-value, of the virus that causes it. It is a measure of how many other people on average an infected person will transmit a pathogen under “normal” conditions, that is, without restrictions.
For a fully effective vaccine, the limits for herd immunity rates for infectious diseases are calculated by dividing 1 by R, subtracting the quotient from 1 and multiplying by 100.
For example, herd immunity against highly contagious measles, with an estimated R-value of 12 or more, will only be achieved if 92% or more within a group are immune. For the seasonal flu strain that can have an R 1.3 value, the limit will be only 23%.
“The problem is that at the moment we do not know exactly how fast the virus spreads without precautions and with the normal levels of travel and social activity that we had a year ago,” said Winfried Pickl, professor of immunology at the Medical University of Vienna. .
With so many countries still operating far from normal, the assumption should be that the R-value for COVID-19 will be “closer to 4 than 2,” he said, adding that even with meters of partial or total blockade, the value of R is around 1.5.
Additionally, any vaccine efficacy below 100%, such as 90% or roughly as seen in early Pfizer-BioNtech and Moderna COVID-19 vaccine data, will require a corresponding increase in coverage to achieve the limit. of herd immunity.
Ames Andalja, an academic at the Johns Hopkins Center for Health Insurance, said a good goal for immunity in the United States would be to vaccinate more than 70 percent of the population, but added that the rate could increase if vaccines were less effective. .
Is transmission to the “herd” stopped?
Experts say another important factor is whether the COVID-19 vaccines the government chooses to develop can stop the spread of the virus.
The evidence so far shows that the first COVID-19 vaccines on the market will at least stop the development of the disease in humans. But it cannot be ruled out that people can still contract the SARS-CoV-2 virus and spread it to other people without realizing it.
“While protection against disease has value to the individual, it will not prevent the spread of the virus and the risk of people becoming ill without vaccination,” said Penny Ward, visiting professor of pharmaceutical medicine at King’s College London.
Bodo Plachter, professor and deputy director of the Institute of Virology at the University Hospital in Mainz, Germany, said respiratory infections in particular can be difficult to prevent completely with vaccines, although vaccines will somehow reduce the number. circulating virus.
“Vaccinated people can shed less virus,” he said. “But it would be a mistake to believe that only vaccination can suppress a pandemic.”
Focus on protecting the vulnerable?
Riley, from the University of Edinburgh, said this means that, for the time being, seeking herd immunity via COVID-19 vaccination will not work.
A better approach, he said, might be to use the first limited doses of the vaccine to protect those most in need, without worrying about the more robust members of the “herd” who may live with the virus.
“Let’s forget about protecting the masses to protect the vulnerable,” he said. “Let us directly protect the vulnerable.”
Source: ΑΠΕ-ΜΠΕ
