(CNN) – This time last year, brand-new, astonishingly effective Covid-19 vaccines rolled across the country, injecting a strong tone of optimism into America’s once-groping pandemic response.
Millions of people stood in line daily to get their shots. Instead of the constant drumming of cases, hospitalizations, and deaths, we tracked a new number: the percentage of Americans who had been vaccinated. This number, we thought, was our best chance of beating the virus.
The United States was caught in a feverish dream of reaching herd immunity, a threshold we could cross where vulnerable individuals – including those who are too young to be vaccinated or those who did not respond well to the vaccines – could still be protected because as a society, we would weave an invisible safety net around them.
With herd immunity, if someone gets infected by a virus, they are surrounded by enough people who are protected from infection that the virus has nowhere to go. It does not succeed in spreading.
As a country, we had reached here against some formidable viruses, such as rubella and measles. We thought we could get there with Covid-19. We were probably wrong.
“The concept of classical herd immunity may not apply to Covid-19,” said Dr. Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases, in an interview with CNN.
And that “means we are not going to be without SARS-CoV-2 in the population for an extended period of time,” said Fauci, who was recently co-author a paper on herd immunity for 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 one 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 found that Omicron viruses are just as contagious as measles.
The United States eliminated the transmission of measles and has successfully prevented the virus from circulating in this country due to three things: an extremely effective vaccine; a virus that does not change or mutate in significant ways over time; and a successful childhood vaccination campaign.
The measles vaccine is 97% effective in preventing the disease, According to the CDC. Once a person is vaccinated, studies have found that protection lasts almost a lifetime.
Many states in the United States had once reached an ambitious public health goal of having more than 90% of their children vaccinated against the disease before starting kindergarten.
This high level of vaccination 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 U.S. borders. Every year, a certain number of cases arise when travelers bring it into the country, but it has never regained a foothold here and has continued to circulate because we have protection against it at the societal level.
Elimination of the virus is not infallible. In the United States, herd immunity to measles is becoming thinner in many parts of the United States – and indeed around the world – due to vaccine hesitation.
World Health Organization warned in 2019 that measles can become endemic again around the world as more people refuse their shots.
Unfortunately, Covid-19 does not play by the same rules.
“The bad news number one,” Fauci said, is that the coronavirus that causes Covid-19 is changing a lot and in significant ways.
“We have already experienced over a period of two years that we have had five separate variants Alpha, Beta, Delta, Omicron. And now BA.2 from Omicron one, ”he said.
“The bad news number two is that there is a lack of broad acceptance of safe and effective vaccines,” Fauci said. In short, not enough people have been vaccinated.
The more contagious the virus, the more people need to be vaccinated to prevent it from tearing through 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 thread and in a recent interview with CNN, Kucharski explained how expectations about herd immunity should 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 were able to prevent 85% of the transmission of the virus.
If the vaccines did not prevent transmission to that extent, he said, herd immunity would probably not be possible with the vaccines we currently have.
In May 2021 paper on the same topic published in the journal Eurosurveillance, Kucharski and his co-authors explain that much of herd immunity also depends on how well the vaccines prevent transmission – the action of one infected person transmitting the virus to another.
Vaccines that prevent transmission are said to confer sterilizing immunity. The measles vaccine creates sterilizing immunity. The Covid-19 vaccines do not. While vaccination reduces the chances of you transferring Covid-19 to another, contact tracking studies have shown it’s still happening.
If not enough people are vaccinated – which is largely the whole population for highly contagious variants – or the vaccines we have do not stop almost all transmission, we may not be able to achieve herd immunity for Covid-19 before they most people have developed immunity after getting 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, but infection-induced immunity is not lifelong,” Fauci said, meaning we will need repeated exposures to either vaccines or infections to keep our defenses up to speed.
However, some are not ready to give up on the idea altogether.
Barry Bloom is Professor Emeritus of Public Health at Harvard University. He says one way to get there would be to make better vaccines.
Companies are working on vaccines that will target more stable areas of the virus, including the tip protein strain, which does not appear to mutate as much. It can create more lasting immunity that could resist the modification of the variants of the viruses.
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 most needed to create the kind of sterilizing immunity that prevents transmission.
If it’s not a vaccine in a nasal spray, Bloom says, why not put monoclonal antibodies in a spray you could take daily before leaving home to prevent virus transmission?
“And the question is, are they good enough to wipe it off? [the virus] out before being transmitted asymptomatically? Or is it a constant game we have to live with? ” Bloom said in an interview with CNN.
Or, Bloom says, perhaps the best we can expect is help from evolution. He says the virus changes to become more contagious over time, but not necessarily to cause more serious illness. In the end, it does not do much good for the virus to kill a person. It needs hosts. It would be much better for the virus to evolve to become as contagious as it can be, but perhaps with less tendency to cause serious illness.
Bloom believes this is probably what happened to the coronaviruses that are now causing the common cold. He thinks they once again started out as violent predators, but evolved over time to just be pests.
That way, they will live on, but so will we.
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