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Cory Franklin and Robert Weinstein, Tribune News Service

“A man has got to know his limitations.” That is one of the memorable phrases uttered by “Dirty Harry” Callahan, the fictional police inspector played by Clint Eastwood. Today, the world is learning its limitations in the ongoing COVID-19 pandemic, and the most important lesson is that a key strategy we banked on to defeat the virus — herd immunity — appears unobtainable.

Herd immunity is reached when enough people in a community acquire immunity to a disease, either through infection or vaccination or both, that it makes sustained transmission impossible and protects even those who are not immune. The strategy of mass vaccination campaigns to achieve herd immunity has worked well for childhood diseases such as diphtheria, which has essentially disappeared, and for other once-common diseases such as smallpox and polio. (The first U.S. case of polio in a decade was recently reported, thought to be brought in from outside the country.)

The percentage of people who must be immune to stop the spread of a disease depends primarily on how contagious the disease is. The commonly cited example is measles, which is extremely contagious and requires immunity in 90% to 95% of the population to halt transmission. In the past few years, measles outbreaks have occurred in the U.S. despite greater than 80% of the local population being vaccinated.

Which brings us to COVID-19. Early on, public health researchers and the World Health Organization estimated that a range of 60% to 70% population immunity would be necessary to control the disease. As the COVID-19 vaccines became available, more contagious variants also emerged. Public health experts were forced to move up the herd immunity estimate to 75% to 80%. But events on the ground have shattered that notion.

What went wrong with the herd immunity theory? In essence, two things. First, vaccine protection is incomplete and does not last long enough. Second, the virus is constantly mutating to circumvent vaccine protection.

The theory of herd immunity, like all scientific theories, depends on several assumptions, and these assumptions proved not to be true in real life. The first fallacy was that people who have acquired immunity would not acquire or pass on the disease; they must remain resistant. One year ago, after a July 4 gathering in Provincetown, Massachusetts, where nearly everyone was vaccinated, about 1,000 people came down with COVID-19. That disproved the assumption that vaccinated individuals could not acquire or transmit COVID-19. We have since learned that vaccine-induced immunity wanes over time, thus prompting the need for booster shots, which themselves wane in effectiveness over time.

Second, herd immunity theory depends on immunity to a relatively stable disease. Measles does not change much from year to year, and the measles vaccine does not have to be reconfigured annually. In the first months of the COVID-19 pandemic, it was believed the virus would take a long time to mutate. But immune-evading variants appeared more quickly than expected in the U.S. and all over the world. Several countries with vaccination rates greater than 90%, such as Portugal, have experienced significant waves of infection this year, largely as a result of new variants. Despite the failure to prevent all infections, vaccination has dramatically lessened the severity of disease and the number of COVID-19 deaths in most countries — no small achievement. This may represent a kind of “partial herd immunity,” but the possibility of new variants and the limitations of the current vaccines represent a continuing worldwide threat.

As Dirty Harry said, we have our limitations — but we are not powerless. Work proceeds on a universal coronavirus vaccine and a nasal vaccine, both of which might be better suited to stopping COVID-19. We should increase our wastewater testing for COVID-19, which may provide a badly needed early-warning system for impending infection surges and new variants. Information technology can better integrate home testing with public reporting to prevent undercounts of cases, and this can also be used to facilitate quick drug treatment with Paxlovid for those who test positive.

A full-scale national evaluation of adequate indoor ventilation in our buildings, especially our schools, is overdue. When case counts are high, masking and social distancing in certain settings are a responsible way to protect the most vulnerable. Finally, because being overweight is such a strong risk factor for severe COVID-19, a national campaign to reduce obesity is in order. It worked for smoking.

In the near term, COVID-19 is not going away. We cannot say now whether it will be an ongoing significant health threat or a minor inconvenience. Realizing how little we control events, we must hope — with humility — that we are lucky and it turns out to be the latter. But preparation is essential because, as baseball executive Branch Rickey observed, luck is the residue of design. Or as Dirty Harry was fond of asking with menace, “Do you feel lucky?”