By Madeline Keleher
May 17, 2021
Initially, scientists estimated we’d reach herd immunity once 60% of the population either had had COVID-19 or been vaccinated. Now, the number has been bumped up and many are saying we won’t reach herd immunity at all. What’s with the moving goal post?
Last October, a libertarian group called the American Institute for Economics Research drafted the Great Barrington Declaration, proclaiming that instead of waiting for vaccines, healthy people should resume their normal lives and become infected with COVID-19 to build herd immunity while retired people should keep having groceries delivered. (This same organization has argued for no regulations on sweatshops, for Brazilians to continue slashing their rain forests, and that air quality is not degrading nor is radon in homes an issue–so, at least they're consistent.) Horrified scientists called the proposal to rapidly ascend to herd immunity dangerous. To see why, let’s crunch the numbers on the proposal. In the United States, COVID-19 has killed 1.8% of people who have contracted it. In our population of 332 million, reaching a herd immunity threshold of 60% before vaccines were available would have required 199,200,000 infections, resulting in approximately 3.6 million deaths. Fortunately, most leaders deemed it too high a price to pay for young people to party at bars, and kept COVID-19 restrictions in place through the winter.
But we are in a very different place now. Multiple vaccines have passed rigorous clinical trials with flying colors and are available for free to any American who wants one. That should mean we will reach herd immunity soon, right? Unfortunately, no, for several reasons.
1) Vaccines are not being distributed equitably around the world.
87% of all COVID-19 vaccines have gone to richer countries, while just 0.2% have gone to low-income countries. This has led to a tremendous imbalance in vaccination rates, with 1 in 4 people in high-income countries already fully vaccinated compared to just 1 in 500 people in low-income countries. At least 90% of people living in low-income countries will not be able to get a vaccine this year. The lowest income groups will likely be waiting two years. Unequal vaccination rates promote the development of COVID-19 variants. The B.1.1.7 variant—which is both more contagious and more deadly than the original strain—may well have originated in an immunocompromised person whose immune system took far longer than normal to clear the virus, allowing it to accumulate mutations. Until vaccination rates are high globally, the virus will continue to mutate—especially in the most vulnerable people—and circulate new variants around the world.
2) Despite having the fortune of living in a country where vaccines are plentiful, not all Americans want a vaccine.
Initially, vaccine access was low and vaccine hesitancy was high in communities of color, whose mistreatment in healthcare has been rampant across history and egregiously continues today. With time and increased outreach, this hesitancy in communities of color has been waning. In fact, vaccine hesitancy has declined in general in the United States, with 39% of adults saying in December 2020 they planned to “wait and see” before getting vaccinated, and just 15% saying so in April 2021.
However, although we are nowhere near herd immunity, demand for vaccines is slowing, and mass vaccination sites are shutting down. The portion of Americans who say they will definitely not get a vaccine has hardly budged since December, dipping slightly from 15% to 13% in four months. A partisan divide is evident, with 5% of Democrats refusing to be vaccinated and 20% of Republicans.
There is still hope that people who are currently hesitant about vaccination may change their minds. Research from a Republican focus group has shown that when communicating with vaccine skeptics, it is important to: focus on the benefits of getting the vaccine, acknowledge fears of vaccine side effects but contrast them with known threats of the virus, and explain that the vaccine was developed in record time due to cutting red tape but not cutting corners. It is crucial to avoid emotional appeals, politics, shaming, judging, and fixating on the risks of not being vaccinated—none of that is productive. If you'd like to help vaccine-hesitant loved ones, don’t tell them what to do, listen compassionately and provide facts.
Evangelical Christians are the leading vaccine hesitant group. While just over half have already been vaccinated, 20% of evangelicals say they will not get vaccinated. Some religious leaders are attempting to persuade fellow Christians that getting vaccinated is at its core a Christian act. For instance, the Christians and the Vaccine project has a series of videos on topics such as if Christians should take the vaccine, how Christians can spot fake news on the vaccine, and if taking the vaccine (like partaking in any aspect of modern medicine) contradicts a pro-life stance. These efforts are important, as we cannot reach herd immunity if large swaths of people continue refusing the vaccine.
3) Even if everyone gets vaccinated, immunity may not last.
Immunity to other coronaviruses fades with time, sometimes in as quickly as one year. Modeling indicates that if COVID-19 immunity lasts two years we can expect outbreaks every other year, whereas we’ll have annual outbreaks if immunity lasts just 10 months. 89% of immunologists expect COVID-19 to become endemic, meaning it will be here to stay (but in a lower steady state with occasional flare ups). Three of the four coronaviruses that are already endemic in humans have likely been around for hundreds of years. It is probable you will need a COVID-19 booster shot next year and possible you will need boosters annually. Fewer than half of Americans get flu shots annually, even though the flu kills tens of thousands of their fellow citizens each year. Convincing people to keep up their COVID-19 immunity to control the disease may also be challenging.
4) New variants continue to push the herd immunity threshold higher.
Herd immunity is calculated by a deceivingly simple fraction: p = 1 – 1/R0. P is the percentage of people who must no longer be susceptible to the disease in order to reach herd immunity. R0 (pronounced “R naught”) is the disease’s basic reproduction number. The higher the value of R0, the higher the value of p required for herd immunity. If a disease has an R0 of 4, then p = 1 – 1/4 = 0.75, meaning a herd immunity threshold of 75%. If the R0 is 6, then p = 1 – 1/6 = 83%.
The reproduction number for viruses isn’t too different for that of people.
As a person, if you have one kid in your lifetime, you’ve “replaced” yourself. On a population level, women must have an average of 2 kids to “replace” themselves and their partners and keep a steady population size. If women have on average more than 2 kids, the population size will grow over time; fewer than 2 kids and the population will shrink. (The new census data show that the average is now 1.6 kids per woman, meaning that without a change in course, the U.S. population size will decrease—already, half the states in the country had more deaths than births last year.)
Diseases operate with similar math. Let’s say one sick person enters a population of healthy individuals: the number of people that person infects is the R0. If the sick person infects just one other person (an R0 of 1), the disease will persist at a slow, steady state. If, however, the sick person makes 3 other people sick (and each of those people makes 3 more people sick, and so on), the disease will spread exponentially (it’s like if women were having 3 kids, the human population would grow exponentially). If a disease infects someone, and that person doesn’t make anyone else sick, the disease will peter out (it's like if women stopped having kids, the population would shrink and we’d have a Children of Men situation on our hands).
The R0 for the seasonal flu is about 1.3. The 1918 flu had an R0 of 1.8 and Ebola’s R0 is between 1.5 and 2.5. The R0 for measles is anywhere between 3.7 and a whopping 203. COVID-19’s R0 is around 2-3, which spurred initial estimates of the herd immunity threshold being around 50-67%.
The R0 is affected by both the disease itself (how sticky is that spike protein? How long can the virus survive while airborne?) and the population the disease is circulating in (how large is the population? How dense? How do the individuals interact?). Different populations can have different R0s for a disease. In the UK, amidst a national lockdown the R0 there dropped to 0.85—and since the value was below 1, the spread was slowing. But then the new B.1.1.7 variant arose with a higher R0 of 1.25. Now that populations are no longer locked down—and in fact the CDC has revised its guidance to no longer require masks in most indoor spaces—the coronavirus R0 will rise again. The B.1.1.7 variant is now the most common form of the virus in the United States, and as it is 60% more contagious, its R0 is likely around 4.8 (60% higher than the R0 of 3 of the original coronavirus strain). This is why scientists have revised the herd immunity threshold up to 80% (p = 1 – 1/4.8 = 79%), and the R0 will likely increase with new variants.
Yes, it is okay for fully vaccinated people to start socializing again; but keep several things in mind. Currently, just 36.7% of Americans are fully vaccinated—a far cry away from 80%. Children, who make up 24% of the Americans, are mostly unvaccinated. And globally, a mere 3.5% of people are fully vaccinated. The CDC has radically altered its mask guidelines, many states dropped their mask mandates months ago, and handwashing has drastically decreased. Vaccine breakthrough cases are exceedingly rare, but they do happen (so far, out of 115 million fully vaccinated Americans, 792 have been hospitalized with symptomatic COVID-19 and 181 have died). So if you are going somewhere indoors where there may be vulnerable, susceptible people, it is still a good choice to wear a mask. You must do so when using public transportation. Encourage (patiently, non-judgmentally) your hesitant loved ones to get vaccinated. Donate to global vaccination efforts if you have the means. And for goodness’ sake, continue washing your hands before eating and after coughing, sneezing, nose blowing, eating, using the bathroom. Like it or not, we are in this—together—for the long haul.
Comments