Can Vaccination and Infection Rates Equal COVID Herd Immunity?

By Carmen Heredia Rodriguez, Kaiser Health News

Wednesday, March 17, 2021 (Kaiser News) – It has been a long, dark winter of covid issues, fueled by a high number of post-holiday cases and the U.S. death toll exceeding 530,000 lives lost. But with three vaccines – Pfizer-BioNTech, Moderna and Johnson & Johnson – now cleared for emergency use in the United States, there appears to be hope that an end to the pandemic is in sight.

A recent analysis by Wall Street research firm Fundstrat Global Advisors fueled this idea, suggesting that as many as nine states were already achieving coveted “herd immunity” status as of March 7, signaling that a return to normalcy was being achieved. close.

“Suspected herd immunity” is “the combined value of infections + vaccinations as% of population> 60%,” noted a tweet from a CNBC anchor based on a more comprehensive analysis from the firm. It got us thinking: does this calculation hold up?

First, do public health experts generally consider herd immunity to be 60%? Furthermore, does current scientific thought equate protection against antibodies generated by past covid infections with the same degree of protection as vaccination?


We decided to find out.

First, a review of collective immunity. Also known as community or population immunity, the term is used to describe the point at which enough people are sufficiently resistant – or have an immune response – to an infectious agent that it has difficulty spreading to d ‘other.

In this explanation, we noted that people generally acquire immunity from either vaccination or infection. For the contagious diseases that have marked modern history – smallpox, polio, diphtheria or rubella – vaccines have been the mechanism by which herd immunity has been achieved.

As the United States moves closer to that point, most health experts warn, it still has a ways to go. Fundstrat’s analysis offered a rosier interpretation. Although the site is located behind a paywall, the graphic has generated buzz on Twitter and in media such as the Daily Caller.

Fundstrat has relied on a variety of sources – in particular, a data scientist and pandemic modeler named Youyang Gu – to determine the level of immunity a state needs to eradicate the covid, said Ken Xuan, head of the pandemic. company data science research. From there, analysts created a graph meant to track the level of covid immunity in each state. They calculated the number by adding the percentage of people believed to have been infected with the virus to the percentage of people who had received the vaccine.


Xuan, who quickly noted that he was not a public health expert, said he and his team followed Gu’s predictions and arrived at 60%, a figure he admits to be a guess.

“The idea would be that we don’t know if 60% is true,” he said. However, if states that have reached this threshold see a sharp decline in covid cases, “then this is the number to watch.”

What about the 60% marker?

Throughout the pandemic, health experts have tended to set the magic number for herd immunity between 50% and 70% – most, including Dr Anthony Fauci, director of the National Allergy Institute. and infectious diseases, leaning towards the higher end of the spectrum.

“I would say 75 to 85 percent should get the shot if you want to have this herd immunity coverage,” he told NPR in December.

Experts we consulted were skeptical of the 60% figure, saying the mechanics of Fundstrat’s analysis were relatively strong but over-simplified.


Ali Mokdad, director of population health strategy at the University of Washington, said the level of immunity needed to achieve this goal can vary due to several factors. “No one knows what herd immunity to covid-19 is because it’s a new virus,” he said.

That said, Mokdad described the use of 60% as “totally bogus”. Data from other communities around the world shows covid outbreaks occurring at or near this level of immunity, he said. Indeed, the city of Manaus in Brazil has seen cases drop for several months, then rise despite three quarters of their inhabitants having already had the virus.

Josh Michaud, associate director of global health policy at KFF, called the 60% assumption “off base.”

And some said that was not even the main point.

Dr Jeff Engel, senior advisor on covid to the Council of State and Territorial Epidemiologists, said the question of collective immunity might not even be relevant because, when it comes to covid, we could never reach it. . The new virus could become endemic, he said, meaning it will continue to circulate like the flu or the common cold. For him, reducing deaths and hospitalizations is more important.


“The concept of collective immunity means that once we hit the threshold, it will go away,” Engel said. “It is not. It is a misconception.

Natural and vaccine immunity – should they be lumped together?

When asked why Fundstrat’s analysis considered the two types of immunity to be equivalent, Xuan replied that this was a guess.

Here is what current science supports.

Those who receive any of the three vaccines available in the United States enjoy a high level of protection against serious illness and death from covid – even after one dose in a series of two injections.

Additionally, people infected and recovered from the virus appear to retain some protection for at least 90 days after testing positive. Immunity may be weaker and decline faster in people who have developed few or no symptoms.

In practice, two experts said that natural and vaccine-induced immunity work the same in the body. This lends credibility to Fundstrat’s approach.


However, some health experts consider the immunity induced by the vaccine to be better than the protection generated by the infection because it may be more robust, Michaud said. Researchers are still investigating whether people who were infected with the virus but exhibited mild or no symptoms generated an immune response as strong as those who developed more severe illness.

In fact, the Centers for Disease Control and Prevention cites unknowns surrounding natural immunity and the risk of falling ill again with covid as reasons for those with the virus to get vaccinated.

“They haven’t been well studied yet,” Engel said, referring to asymptomatic people. “And maybe we’ll find out that a lot of them haven’t developed really robust immunity.”

Both types of viral protection leave room for groundbreaking potential infections, Michaud said. Neither offers “perfect immunity,” he said. And the jokers remain. How long do the two types of immunity last? How do the systems of different people react? How well will people be protected against emerging variants of the coronavirus?

“It’s a witches brew of different factors to consider when trying to estimate herd immunity at this point,” Michaud said.

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Jothi Venkat

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