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."