Can vaccination and infection rates add up to achieve Covid stove immunity?

It was a long, dark winter filled with concern, fueled by high numbers of post-holiday cases and the American death toll of more than 530,000. However, with three vaccines – Pfizer-BioNTech, Moderna, and Johnson & Johnson – now approved for use in the US emergency, there seems to be hope that the end of the pandemic is in sight.

A recent analysis by Wall Street research firm Fundstrat Global Advisors spurred this idea on, indicating that as of March 7th, nine states had achieved coveted herd immunity status, indicating a return to normal is imminent.

“Suspected ‘herd immunity’ is ‘the combined score of infections + vaccinations as% population> 60%'” noted a Tweet from a CNBC anchor based on a more complete analysis of the company. That got us thinking: Does this calculation hold?

First, do public health experts consider herd immunity to be 60% in general? Does current scientific thinking equate protection against antibodies produced by previous covid infections with the same level of protection as vaccination?

We decided to find out.

First, a herd immunity check. Also known as community or population immunity, the term describes the point at which enough people are sufficiently resistant to an infectious agent or have an immune response that is difficult to transmit to others.

In this statement, we noted that people generally gain immunity through either vaccination or infection. For infectious diseases that have shaped modern history – smallpox, polio, diphtheria, or rubella – vaccines were the mechanism by which herd immunity was achieved.

As the United States approaches this point, most health professionals are warning, but there is still cause for cover. Fundstrat’s analysis offered a rosier attitude. Even though the site is behind a paywall, the diagram generated buzz on Twitter and in news outlets like the Daily Caller.

Fundstrat drew on a variety of sources – notably a data scientist and pandemic modeler named Youyang Gu – to determine what level of immunity a state needs to eradicate Covid, said Ken Xuan, director of data science research for the company. From there, the analysts created a chart to track the level of covid immunity in each state. They calculated the number by adding the percentage of people estimated to have been infected with the virus to the percentage of people who had received the vaccine.

Quickly realizing that he is not a public health expert, Xuan said that he and his team followed Gu’s predictions and came out 60%, a figure he acknowledges is an assumption.

“The idea is we don’t know if 60% is true,” he said. However, if states that have reached this threshold are seeing a sharp drop in Covid cases, this is the number to watch.

What about the 60% marker?

During the pandemic, health professionals tended to set the magic number for herd immunity between 50% and 70% – with most, including Dr. Anthony Fauci, the director of the National Institute for Allergies and Infectious Diseases, approached the high end of The Spectrum.

“I would say 75 to 85% would have to be vaccinated if you wanted that blanket of herd immunity,” he told NPR in December.

The experts we consulted were skeptical of the 60% figure and said the mechanisms of Fundstrat analysis were relatively solid but oversimplified.

Ali Mokdad, chief strategy officer for population health at the University of Washington, said the level of immunity required to achieve this goal can vary based on several factors. “Nobody knows what herd immunity is to Covid-19 because it is a new virus,” he said.

Even so, Mokdad described the use of 60% as “completely wrong”. Data from other communities around the world show covid outbreaks occur at or near these immunity levels, he said. In the Brazilian city of Manaus, cases fell for several months and then rose, even though three-quarters of residents were already infected with the virus.

Josh Michaud, assistant director of global health policy at KFF, described the 60% assumption as “off-base”.

And some said it wasn’t even the main point.

Dr. Jeff Engel, senior advisor on Covid at the Council of State and Territorial Epidemiologists, said the herd immunity issue may not even be relevant as we may never get to it when it comes to Covid. The novel virus could become endemic, he said, which means it will keep circulating like influenza or the common cold. It is more important to him to reduce deaths and hospital stays.

“The concept of herd immunity means that once we reach the threshold, it will go away,” said Engel. “That is not the case. That is a wrong idea.”

Natural and Vaccine Immunity – Should They Merge?

When asked why the Fundstrat analysis treated the two types of immunity as equivalent, Xuan said it was an assumption.

Here’s what current science supports.

Those who receive any of the three vaccines available in the US enjoy high levels of protection from serious illness and death from Covid – even after a dose of a two-shot series.

In addition, people who are infected and recovered from the virus appear to retain some protection for at least 90 days after testing positive. Immunity may be lower and decline more quickly in people who developed few to no symptoms.

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

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

In fact, the Centers for Disease Control and Prevention cite the unknowns related to natural immunity and the risk of getting sick again with Covid as reasons for those who have had the virus to get a vaccine.

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

Both types of virus protection leave room for potential breakthrough infections, Michaud said. Neither of these offers “perfect immunity,” he said. And wildcards remain. How long do both types of immunity last? How do the systems of different people react? How protected will people be from emerging coronavirus variants?

“It’s a witch’s brew, with several factors to consider when trying to gauge herd immunity at this point,” Michaud said.

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