Experts Say COVID-19 Herd Immunity Is Becoming Increasingly Unlikely in the US

Although COVID-19 vaccination efforts are continuing across the country, experts say herd immunity is becoming less likely due to uneven vaccine roll-out, distribution disparities, and continuous virus mutation.

Herd immunity can be achieved through both vaccination rates and immunity gained from surviving COVID-19. Earlier in the pandemic, experts believed that once large parts of the population started being vaccinated, herd immunity would usher in the end of the pandemic, according to an article published by Nature. Most scientists estimated that between 60% and 70% of the population would need to be immune to reach that threshold, either through vaccination or past exposure to the virus.

According to the article, however, experts believe achieving herd immunity in the United States is becoming unlikely due to high levels of vaccine hesitancy, the emergence of new variants, and the delayed arrival of vaccinations for children. Because of these issues, experts now say COVID-19 could become a long-term endemic threat, similar to influenza.

Despite the encouraging progress made over the past few months of the vaccine roll-out, experts say they still lack clarity over whether the vaccines prevent transmission. The key to herd immunity, according to the article, is having too few susceptible hosts to maintain transmission, even if a person becomes infected. If vaccines do not prevent transmission, however, this could make herd immunity nearly impossible.

“Herd immunity is only relevant if we have a transmission-blocking vaccine,” said Shweta Bansal, MS, PhD, in the article. “If we don’t, then the only way to get herd immunity in the population is to give everyone the vaccine.”

Despite these concerns, the article noted that a vaccine doesn’t need to block 100% of transmission in order to be effective. Even 70% efficacy could be a gamechanger, although there would still be a significant amount of transmission that would make it more difficult to break transmission chains.

Disparities in vaccine roll-out also present significant barriers to herd immunity, both globally and within the United States. Some states, such as Georgia and Utah, are lagging far behind others in the percentage of the population that is vaccinated. Furthermore, although all adults are not eligible across the country, a vaccine approval for children is still in the works, leaving children vulnerable.

Although Pfizer and BioNTech have applied for an FDA emergency use authorization to vaccinate children aged 12 years and older, this still leaves a large group unvaccinated. According to the article, approximately 24% of people in the United States are under 18 years of age. If most of this group cannot be vaccinated, then 100% of adults would need to be vaccinated in order to achieve 76% immunity in the population.

Finally, the emergence of new variants is a major concern for experts looking toward herd immunity. Current vaccine roll-out plans are racing against the variants, attempting to vaccinate enough people before the virus can mutate and spread.

Even as the number of vaccinated people grows, experts say higher rates of immunity can put pressure on the variants, leading to mutations that are able to infect even immunized people. Vaccinating quickly can prevent new variants from spreading widely, although the uneven rollout could also give variants more time to develop.

Despite these concerning obstacles to achieving herd immunity, experts say it is important not to forget the improvements that have been made thus far. Preventing severe disease and death is vital, and the currently available vaccines have shown significant efficacy. Although transmission may never stop completely, the prominence of COVID-19 is likely to decrease, allowing a return to some form of normalcy.

REFERENCE

Aschwanden C. Five reasons why COVID herd immunity is probably impossible. Nature; March 18, 2021. https://www.nature.com/articles/d41586-021-00728-2. Accessed May 7, 2021.