Why a 70% COVID-19 vaccination rate doesn't ensure herd immunity – Modern Healthcare

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As COVID-19 inoculations accelerate, a weary public waits anxiously for vaccines to reach 70% of the population, a level widely associated with “herd immunity.”

It’s not that simple. To neutralize a virus that has killed nearly 5,000 Chicagoans and sickened 250,000, immunity must spread across each of the city’s neighborhoods—with a higher concentration in areas that have been hit harder during the pandemic. In other words, vaccinating any mix of 1.9 million residents won’t necessarily stop COVID-19.

“Even if the overall average is showing we’re at 70% or 80%, or we’re getting close to that in a particular subset, that doesn’t mean we have herd immunity that is really going to be protecting the entire population,” says Dr. Rachel Rubin, who co-leads the Cook County Department of Public Health. “You have to look at it within the context of the disease and within the context of certain demographic characteristics related to who is immune and who isn’t.”

Reaching that 70% threshold statewide or even citywide isn’t a guarantee against future outbreaks. If vaccination rates lag in some areas, the virus could continue to circulate and mutate, generating new variants impervious to vaccines and making the entire region vulnerable again.

With a mutating virus, it becomes “much harder to get to herd immunity, and it requires a different public health response than what we typically think of for measles or some other childhood vaccine-preventable diseases,” says Dr. Marielle Fricchione, a medical director at the Chicago Department of Public Health.

The concept of herd immunity is simple: Once a certain percentage of the population becomes immune—either through vaccination or prior infection—viruses are less likely to infect new people. The more easily a pathogen spreads, the higher that percentage needs to be. Experts estimate that the immunity threshold for the coronavirus is somewhere between 70% and 90%.

But in communities where physical distancing at home or at work is more difficult, the threshold is higher than in areas where people can easily isolate and get testing and treatment. For a number of reasons, such as limited access to online scheduling systems and transportation barriers, communities that need to reach higher levels tend to have the hardest time getting vaccinated.

For example, 4.4% of the population that resides in ZIP code 60621—which includes Englewood, one of 15 high-need communities identified by Chicago’s COVID vulnerability index—had been fully vaccinated as of March 17. But the area has one of the highest mortality rates per 100,000 people in the city. Meanwhile, 14.1% of Lincoln Park residents in ZIP code 60614—where the mortality rate per 100,000 people is much lower—have been fully vaccinated.

Communities that have suffered more are also the ones that are still at risk for ongoing transmission, says Dr. David Dowdy, an infectious disease epidemiologist at Johns Hopkins Bloomberg School of Public Health. “So it’s not just about doing what’s fair, although that’s part of it,” he says. “It’s also about doing what’s going to get transmission rates down the fastest.”

Without enough vaccine to inoculate everyone at once, local public health departments have focused on getting early doses to communities with high rates of infection and death.

The city of Chicago got about 65,000 first doses the week of March 15, according to data from the Centers for Disease Control and Prevention.

“A limiting factor for us is how many vaccines we have,” Fricchione says. “No matter how effective they are, if we don’t have enough, we have to really put them in the right place to break chains of transmission.”

That’s one reason the city and other jurisdictions have prioritized seniors in their vaccination campaigns. Regardless of underlying medical conditions, vaccinating older adults has the biggest impact on preventing COVID-19 deaths, according to the Chicago Department of Public Health.

Based on Chicago data, the city prevents one COVID-19 death by vaccinating 49 people who are 80 and older, compared with preventing one COVID-19 death by vaccinating 17,000 people age 18 to 29.

“You have to think about, if any one person—an average person in your community—gets COVID, how many other people are they going to transmit the disease to before they themselves get better,” Dowdy says.

In addition to targeting communities with high rates of infection and death, health officials are focused on reaching communities with large numbers of essential workers and crowded housing, among other indicators of social vulnerability, Fricchione says.

“Those are the communities where one dose of vaccine has a much bigger impact on the chain of transmission,” Fricchione says.

Adding another layer of complexity is the fact that herd immunity is not a steady state—particularly not in the case of COVID-19, since the exact duration of immunity conferred by vaccination and natural infection is still unknown.

“We only have a small window of known strong immunity—that’s basically a three-month window to get as many vaccines into the highest-risk people as we can to try to prevent a third wave of this disease because we know it’s mutating and we know there are variants,” Fricchione says.

Undetected, asymptomatic cases of COVID make it impossible to know the exact percentage of any given population that has some level of immunity. And not all individuals will mount the same immune response, which means some people will be better protected than others.

Even in the case of the highly contagious measles virus, the risk of which remains low in the U.S. due to high rates of immunity, outbreaks have sparked in recent years with the virus penetrating undervaccinated communities.

“Getting vaccine first to the highest risk settings has the biggest impact in driving down case rates,” Chicago Department of Public Health Commissioner Dr. Allison Arwady said during a March 17 press briefing. “It helps protect every one of us from the emergence of variants, and it moves us closer to reopening and getting back to normal.”

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