Epidemic, endemic and herd immunity... Where is covid-19 going?
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(CNN) --
This time last year, surprisingly effective new Covid-19 vaccines were rolling out across the country, injecting a strong note of optimism -- due to expected herd immunity to the virus. -- in the previously bumbling response to the pandemic in the United States.
Millions of people queued daily to get vaccinated.
Instead of the steady pace of cases, hospitalizations and deaths, we were tracking a new number: the percentage of Americans who had been vaccinated.
This number, we believed, was our best chance of beating the virus.
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The United States was caught up in the dream of achieving herd immunity, a threshold we could cross where vulnerable individuals -- including those too young to be vaccinated or those who did not respond well to vaccines -- could be protected from anyway because, as a community, we would weave an invisible safety net around it.
With herd immunity, if someone gets infected by a virus, they are surrounded by enough people that they are protected from infection and the virus has nowhere to go.
It does not spread.
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As a country, we've gotten to this point against some formidable viruses, like rubella and measles.
We think we could get there with covid-19.
We are probably wrong.
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"The concept of classic 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're not going to be without SARS-CoV-2 in the population for any considerable period of time," said Fauci, who recently co-authored an academic 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 is spread through the air.
It is so contagious that if one person has it, 9 out of 10 people around them will get it if they are not immune to it, according to the CDC.
Some experts have estimated that the omicron variants of covid-19 are as contagious as measles.
The United States eliminated measles transmission and has managed to keep the virus from circulating in the country thanks to three things: an extremely effective vaccine;
a virus that does not change, or mutates, significantly over time;
and a successful childhood vaccination campaign.
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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 virtually a lifetime.
In the past, many US states have met the ambitious public health goal of having more than 90% of their children vaccinated against the disease by the time they start kindergarten.
This high level of vaccination coverage, the durability and efficacy of the vaccine, and the relative stability of the virus have helped the United States avoid major outbreaks of the disease for more than 20 years.
Still, herd immunity has to be extended beyond US borders.
There are a number of cases every year when travelers bring it into the country, but it has never taken hold here again or continued to circulate, because we have community-level protection against it.
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Virus removal is not foolproof.
In the United States, herd immunity against measles is running out in many parts of the country -- and, indeed, around the world -- because of vaccine hesitancy.
The World Health Organization warned in 2019 that measles could once again become endemic around the world as more people refuse their vaccinations.
Cornering the coronavirus
Covid-19, unfortunately, is not playing by those same rules.
"The number one bad news," Fauci said, is that the coronavirus that causes Covid-19 changes a lot and in significant ways.
"We've already experienced over a two-year period that we've had five separate variants alpha, beta, delta, omicron. And now the BA.2 of omicron," he said.
"The second bad news is that there is not wide acceptance of safe and effective vaccines," Fauci said.
Simply put, not enough people have been vaccinated.
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The more contagious the virus, the more people need to be vaccinated to prevent it from sweeping 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 a 2021 Twitter thread and in a recent interview with CNN, Kucharski explained how expectations around herd immunity have to change as viruses become more contagious.
Kucharski estimated that for a virus as contagious as the delta variant of covid-19, 98% of the population would have to be vaccinated if the vaccines we have could prevent 85% of virus transmission.
If vaccines don't prevent transmission to that extent, he said, then herd immunity probably wouldn't be possible with the vaccines we currently have.
In an article on the same topic published in May 2021 in the journal Eurosurveillance, Kucharski and his co-authors explain that much of herd immunity also depends on the effectiveness of vaccines in preventing transmission, i.e. the fact that an infected person transmits the virus to another.
Vaccines that prevent transmission are said to transmit sterilizing immunity.
The measles vaccine creates a sterilizing immunity.
The covid-19 vaccines do not.
Although vaccination reduces the chances that covid-19 can be transmitted to another person, contact tracing studies have shown that it still occurs.
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If not enough people are vaccinated -- which has to be practically the entire population in the case of highly contagious variants -- or the vaccines we have don't stop almost all transmission, we may not be able to achieve immunity from herd for Covid-19 until most people have developed immunity after contracting the infection, Kucharski wrote in the article.
Other factors must also be taken into account, 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're going to need repeated exposures to either vaccines or infection. to keep our defenses up to date.
Keeping hope on herd immunity
However, some are unwilling to give up the idea altogether.
Barry Bloom is an emeritus professor of public health at Harvard University.
He says that one way to do this would be to make better vaccines.
Companies are working on vaccines that target more stable parts of the virus, including the spike protein stem, which doesn't seem to mutate as much.
This could create a longer-lasting immunity that could resist the shape-shifting variants of the virus.
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There are also promising vaccines in nasal spray form that could help develop antibodies in the nose and throat.
The hope is that these vaccines can generate immunity in the tissues that are 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 that can be taken daily before leaving the house to prevent transmission of the virus?
"And the question is are they good enough to kill it [the virus] before it spreads asymptomatically? Or is it a constant game that we're going to have to live with?"
Bloom said in an interview with CNN.
Or, according to Bloom, perhaps the best we can hope for is help from evolution.
He says the virus is changing to be more contagious over time, but not necessarily to cause more severe illness.
Ultimately, killing a person doesn't do the virus much good.
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It would be much better if the virus evolved to become as contagious as possible, but perhaps less likely to cause serious illness.
Bloom thinks this is probably what happened with the coronaviruses that now cause common colds.
He thinks they probably started out as ferocious predators, but over time they evolved into simple pests.
In this way, they continue to live, but so do we.
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