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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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