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Is collective immunity against Covid-19 still possible?

Millions of people lined up every day to get vaccinated. Instead of the regular drumbeat of cases, hospitalizations and deaths, we were tracking a new number: the percentage of Americans who had been vaccinated. That number, we believed, was our best chance of beating the virus.

The United States has been caught in a fever dream of reaching herd immunity, a threshold we could cross where vulnerable people – including those too young to be vaccinated or those who have not responded well to vaccines – could be protected anyway because, as a community, we would weave an invisible safety net around them.

With herd immunity, if someone is infected with a virus, they are surrounded by enough people protected from infection that the virus has nowhere to go. It fails to spread.

As a country, we had reached this point against some dreaded viruses, such as rubella and measles. We thought we could do it with the Covid-19. We were probably wrong.

“The classic herd immunity concept may not apply to Covid-19,” Dr. Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases, said in an interview with CNN.

And that “means we’re not going to go SARS-CoV-2-free in the population for any significant period of time,” said Fauci, who recently co-authored a paper on herd immunity for the Journal of Infectious. Diseases.

How we beat measles

Fauci points to measles as an ideal case study in herd immunity.

Like the virus that causes Covid-19, the measles virus spreads through the air. It is so contagious that if a person has it, 9 out of 10 people around them will catch it if they are not immune to it, according to the CDC. Some experts have estimated that Omicron viruses are as contagious as measles.

The United States eliminated the transmission of measles and succeeded in preventing the virus from circulating in this country thanks to three things: a highly effective vaccine; a virus that does not change or mutate significantly over time; and a successful childhood vaccination campaign.

The measles vaccine is 97% effective at preventing the disease, according to the CDC. Once a person is vaccinated, studies have determined that the protection lasts virtually a lifetime.

Many US states had already achieved an ambitious public health goal of having more than 90% of their children vaccinated against the disease by the time they started kindergarten.

This high level of vaccine coverage, the durability and effectiveness of the vaccine, and the relative stability of the virus have helped the United States prevent major outbreaks of the disease for more than 20 years.

Yet herd immunity must extend beyond the borders of the United States. Every year there are a number of cases when travelers bring it into the country, but it never found a foothold here and kept circulating because we have community protection against it.

Eliminating the virus is not foolproof. In the United States, herd immunity to measles is waning in many parts of the United States — and indeed around the world — due to vaccine hesitancy.
The World Health Organization warned in 2019 that measles could become endemic again around the world as more and more people refuse their vaccines.

Corraling Covid

Covid-19, unfortunately, does not follow these same rules.

“The number one bad news,” Fauci said, is that the coronavirus that causes Covid-19 is changing a lot and significantly.

“We’ve already experimented over a two-year period that we have five distinct Alpha, Beta, Delta, Omicron variants. And now BA.2 of Omicron one,” he said.

Covid-19 cases are trending up again in the US, driven by BA.2 growth

“The second bad news is that there isn’t wide acceptance of safe and effective vaccines,” Fauci said. Simply put, not enough people have been vaccinated.

The more contagious the virus, the more people need to be vaccinated to prevent it from spreading in a community, according to Dr Adam Kucharski, co-director of the Center for Epidemic Preparedness and Response at the London School of Hygiene and Tropical Medicine. In July 2021 Twitter feed and in a recent interview with CNN, Kucharski explained how expectations about herd immunity must change as viruses become more contagious.

Kucharski estimated that for a virus as contagious as the Delta variant, 98% of the population would need to be vaccinated if the vaccines we have could prevent 85% of virus transmission.

If vaccines didn’t prevent transmission to this extent, he said, then herd immunity probably wouldn’t be possible with the vaccines we have now.

In a May 2021 article on the same subject published in the journal Eurosurveillance, Kucharski and his co-authors explain that a large part of herd immunity also depends on the ability of vaccines to prevent transmission – the act of an infected person transmitting the virus to someone else.
Covid-19 "super spread" can still happen, but now we have the tools to slow it down
Vaccines that prevent transmission are said to transmit sterilizing immunity. The measles vaccine creates sterilizing immunity. Covid-19 vaccines do not. Although vaccination reduces the chances of transmitting Covid-19 to someone else, contact tracing studies have shown that it still happens.

If not enough people are vaccinated – which must be almost the entire population for highly contagious variants – or if the vaccines we have do not stop almost all transmission, we may not be able to reach the herd immunity to Covid-19 until most people developed immunity after contracting the infection, Kucharski wrote in the article.

There are also other factors to consider, such as the durability of immunity over time.

“Not only is vaccine-induced immunity not lifelong, infection-induced immunity is not lifelong,” Fauci said, which means we’re going to need repeated exposures. vaccines or infection to keep our defenses up to date.

Maintain hope

However, some are not ready to completely give up on the idea.

FDA vaccine advisers say plan to update Covid-19 vaccines needed

Barry Bloom is Emeritus Professor of Public Health at Harvard University. He says one way to do that would be to make better vaccines.

The companies are working on vaccines that would target more stable regions of the virus, including the spike protein stem, which doesn’t appear to mutate as much. This could create longer-lasting immunity that could resist the shape-shifting of virus variants.

There are also promising nasal spray vaccines that can help develop antibodies in the nose and throat. The hope is that these vaccines can generate immunity in the tissues they need most to create the kind of sterilizing immunity that prevents transmission.

If it’s not a vaccine in a nasal spray, says Bloom, why not put monoclonal antibodies in a spray you could take daily before you leave the house to prevent transmission of the virus?

“And the question is are they good enough to wipe it [the virus] before they are transmitted asymptomatically? Or is it a constant game we’re going to have to live with?” Bloom said in an interview with CNN.

Or, says Bloom, perhaps the best we can expect is some help from evolution. He says the virus evolves to become more contagious over time, but not necessarily to cause more severe disease. In the end, killing a person doesn’t do the virus much good. He needs guests. It would be much better if the virus evolved to become as contagious as possible, but perhaps with less propensity to cause serious illness.

Bloom thinks that’s probably what happened to the coronaviruses that now cause colds. He thinks they probably started out as ferocious predators, but evolved over time to be just pests.

This way they can live, but so can we.


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