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Suddenly, The Shine Is Off Herd Immunity

Expectations Are Changing With New Thinking

Returning To Pre-Pandemic Normal Life No Longer An Option

Throughout the COVID-19 pandemic, epidemiologists and other scientists have proposed different thresholds as target levels we should aim for to achieve herd immunity.  The notion being put forth was that once this level of immunity was reached, everyone in the population would be protected because the virus would be unable to find enough susceptibles to spread any further. Thus, as described in a recent Slate article by Malia Jones, “the phrase ‘herd immunity’ has become our utopia—a promised land where we can live life as it was before the pandemic.”

Not Clear Cut

Now many observers are examining the concept of herd immunity more closely and coming to the conclusion that the concept is not as clear cut or simple as has been portrayed by many science spokespersons in the media. Even the threshold levels have changed. These were initially thought to be between 60-70% and are now being set closer to 80% and even 90% by at least one investigator.

If for no other reason, these higher thresholds should serve to warn that attaining such high levels will be nearly impossible. But as many observers have now noted, there are plenty of other reasons why we should stop thinking about achieving herd immunity as a magic moment when everyone is protected and things can return to normal.

Stanford University epidemiologist Julie Parsonnet told local media “Herd immunity is a nice idea but in reality it’s a concept best applied to cow herds—or perhaps to nursing homes, ships, boarding schools, or islands but not to an entire country or the world.”

Questioning Herd Immunity

Among the reasons to question the feasibility of herd immunity are:

The transmissibility of a disease will impact the possibility of reaching herd immunity. As reported in a guest essay by Erin Mordecai and colleagues in the New York Times, transmissibility is not fixed and can change based on characteristics of the population or its behaviors.

Also, the virus characteristics can change and the virus can become more transmissible as has occurred with the SARS-CoV-2 variants which have emerged.

World Situation

The US does not exist in isolation. The virus is circulating in other countries and the prospect of manufacturing enough vaccine and distributing it wide enough to protect the world population is years away, if it can ever be achieved. In the meantime, the virus will continue to circulate and may produce new variants which can threaten any protection level achieved in the US or elsewhere.

Vaccination Feasibility

Another reason to question herd immunity is the percentage of adults who are vaccine hesitant or refuse to be vaccinated is unlikely to decrease to negligible levels. For example, the concept of herd immunity itself may be causing people to reject vaccination because they are waiting for others to get vaccinated and create a safe zone for the unvaccinated. This is the “free rider” problem in which some persons benefit from public goods without having to pay for them.

And there will always be pockets of susceptibles where low coverage exists. Just as there may be a herd immunity threshold there is also a maximum feasible level of the population that can be vaccinated in a free society.

Endemicity

Herd immunity, at whatever level may be required, will not be enough to eradicate the virus as most experts expect the virus will become endemic rather than disappear completely. As described during a panel discussion about herd immunity at Stanford University, it is likely that “there will be a plateau in infection with a decrease in prominence over time but continued circulation. It will not be as prominent but will continue to circulate. There will be no clear cut endgame. This endemic situation will require maintenance via ongoing vaccination because immunity will wane and new susceptibles will be constantly entering the population.

Level of Acceptability

Some experts see a solution in calling for a more targeted rather than universal approach to vaccination. This would involve focusing on vaccination of people who are most likely to have an impact on the level of disease at the overall population level. This would include persons who can transmit the virus such as young adults with multiple contacts, people who live in multigenerational households, or people most like to be hospitalized or die from COVID-19. This more targeted vaccination approach seeks not to ever eliminate the virus completely or keep it totally out of the population but to reach a level of virus circulation and virus impact that is acceptable even if not totally satisfactory. In short, there may be a level of COVID endemicity that populations are willing to tolerate or have to tolerate given the realities of what is achievable.  ■

 


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