The world has been jealous of the Covid-19 vaccination campaign in the US for several months now. More than half of the adults in the country have now been vaccinated with at least one dose of a vaccine. But daily vaccination coverage is declining, and there is a broad consensus among scientists and public health experts that the herd immunity threshold is not achievable – at least not in the foreseeable future, and perhaps never.
The shift in outlook poses a new challenge to public health authorities. The urge for herd immunity – by summer, some experts once thought possible – captured the imagination of large sections of the public. Saying the goal won’t be achieved adds one more entry to the list of reasons vaccine skeptics use to avoid getting vaccinated.
Dr. Anthony Fauci, the Biden administration’s chief adviser for Covid-19, recognized the shift in the experts’ thinking last weekend. So they have come to the conclusion that instead of making a long-promised exit, the virus will most likely become a manageable threat that will continue circulating in the United States for years to come and still cause hospitalizations and deaths, much less so extent.
How much less is uncertain and depends in part on how much of the US and the world is vaccinated and how the coronavirus is evolving. However, it is already clear that the virus is changing too quickly, new variants spread too easily, and vaccination is too slow to have herd immunity quickly within reach.
Herd immunity from 60 to 70 percent already to 80 percent or more
As the new coronavirus began to spread around the world in early 2020, it became increasingly clear that the only way out of the pandemic would be for so many people to gain immunity – either through natural infection or vaccination.
Early on, the target herd immunity threshold was estimated to be about 60 to 70 percent of the population. Most experts expected that we would be able to achieve that as soon as vaccines became available.
But as vaccines were developed and their distribution increased, estimates of that threshold began to rise. That’s because the initial calculations were based on the infectivity of the original version of the virus. The predominant variant now circulating among us, but also in the United States, B.1.1.7, first identified in Great Britain, is about 60 percent more transmissible.
As a result, experts now calculate the herd immunity threshold at at least 80 percent. If more infectious variants develop, or if scientists discover that immunized humans can still transmit the virus, the calculation will have to be revised upwards.
1 in 3 Americans don’t want to get vaccinated, but that’s not the only problem
Polls show that about 30 percent of the U.S. population is still reluctant to get vaccinated. That number is expected to improve, but probably not enough. And while that’s a major reason the United States is unlikely to achieve herd immunity, it’s not the only reason.
Herd immunity is often described as a national target. But that’s a vague concept in a country the size of the US. If coverage is 95 percent in the United States as a whole, but 70 percent in a small town, the virus will make its way to the small town. How isolated a particular region is from the coronavirus depends on a dizzying array of factors. Plus, given the degree of movement between regions, a small wave of virus in a region with a low vaccination level can easily spread to an area where the majority of the population is protected.
At the same time, connectivity between countries, especially as travel restrictions diminish, underscores the urgency to protect not just Europeans or Americans but everyone in the world. All variants that emerge in the world will eventually reach Europe or the United States.
Local elimination is the new goal
And many parts of the world are far behind the United States in terms of vaccinations, as well as Europe. Less than 2 percent of people in India are currently fully vaccinated and in South Africa less than 1 percent.
If the herd immunity threshold proves not to be attainable, the most important thing is to control hospitalizations and deaths after pandemic restrictions have been relaxed. By focusing on vaccinating the most vulnerable, the United States has already slashed that number. If the vaccination coverage of that group continues to rise, it is expected that the coronavirus could become seasonal over time, like the flu, and will mainly affect the young and healthy people.
In the long run – a generation or two – the goal is to ‘transfer’ the new coronavirus to become more like its cousins who cause colds. That would mean that the first infection occurs early in childhood and subsequent infections are mild due to partial protection, even if immunity wanes.
But eradication of the virus is impossible at this stage. Local elimination is the new goal. If communities continue to vigilantly test and monitor, it may be possible to keep the number of new cases so low that health officials can identify any new introduction of the virus and immediately quell a potential outbreak. The strategy is described in an article published Thursday in the scientific journal The Lancet.
Children will also have to be vaccinated
The end goal has changed, but the most pressing challenge remains the same: keep vaccinating to stay above the threshold. The skepticism of many Americans about the vaccines and the lack of access to those vaccines by some populations – homeless people, migrant workers, communities of color – make it challenging to reach that goal.
Vaccine mandates would only make that attitude worse, some experts believe. People often need to see others in their social circle embrace something before they are willing to try. Emphasizing the benefits of vaccination for their personal lives can be more motivating than the vague idea of herd immunity.
Although children spread the virus less efficiently than adults, the US experts all agree that vaccinating children will also be important to keep the number of Covid cases low.