Herd Immunity and Vaccine Equity: How We Look at Vaccine Coverage in Different Ways

COVID-19 vaccinations began in Dane County on December 14, 2020, and shortly afterwards we began presenting vaccination data on our dashboard and in our data snapshots. Most of the vaccination data we present, in alignment with the Wisconsin Department of Health Services (DHS), is based on the population as a whole, and not restricted to only those who are eligible for the vaccine. Yet for the past several weeks, we’ve also been including a graph in our data snapshots that shows the percent of the eligible population (i.e., age 16+) that has received a vaccine. For example, in the data snapshot that will be released today, page one will show that 55.3% of the entire Dane County population has received at least one dose of vaccine (along with several other measures based on the entire Dane County population), while page three will show that 67% of the eligible Dane County population has received at least one dose of vaccine.

The percent of the entire population that has received a vaccine is important to understand population immunity.

Herd immunity is achieved when enough people in a population have immunity (by vaccination or previous infection) to prevent the continued spread of COVID-19 throughout the population. However, once a herd immunity threshold is reached, that does not mean the pandemic will suddenly be over. The infection rate (R) will continue to decrease, meaning that each person who is or becomes infected will go on to infect fewer and fewer people, but infections will still occur, particularly within groups that have a lower level of immunity (such as children). It can still take some time before cases will get to a very low level.

We do not yet know what level of vaccination leads to herd immunity for COVID-19 or how current or future variants might affect herd immunity. We know, based on other diseases, that herd immunity is likely at least 60%, and the threshold may be higher with the increasing prevalence of the more transmissible B.1.1.7 variant of concern, along with other circulating variants. Therefore, our current target range is 60-90% immunity among the entire Dane County population. It’s important to look at the entire proportion of the population that has immunity because everyone who does not have immunity, such as children under the age of 16 who aren’t yet eligible for a vaccine and have not contracted COVID-19, can be infected and spread the virus on to others.

Looking at overall vaccination rates among various subgroups can also tell us about how vulnerable each group might still be to COVID-19 infection. If, for example, 70% of the county as a whole has been vaccinated, but some municipalities within the county have a rate of only 40%, there is still a large potential for outbreaks to occur in those communities.

The percent of the eligible population that has received a vaccine is important to understand vaccine equity, access, and uptake.

Looking at the percent of people within subgroups who have received a vaccine among those who are eligible can tell us more about inequities in vaccine access. For example, the following chart shows the percent of each group that is under the age of 16 and therefore ineligible to receive a vaccine:

Percent of Dane County residents in each group who are under the age of 16 and therefore currently not eligible to receive a vaccine:

A larger proportion of those under the age of 16 are people of color compared to those age 16 and older, which explains at least part of the reason why smaller proportions of populations of color have received a vaccine compared to white people. There can be complex factors, such as transportation, language barriers, work in occupations that don’t allow for time off to get a vaccine, or healthcare distrust that may prevent someone from getting vaccinated against COVID-19. Additionally, these data are affected by who was first eligible for vaccine—people of color may be underrepresented in different professions like health care.

When presenting the percent among those eligible within each group that has received at least one dose of vaccine, inequities still exist:

Percent of currently eligible Dane County residents in each group who have received at least one dose of vaccine*:
 

*Note: 11.8% of people vaccinated have a race listed as Other/unknown, and 10.5% are missing ethnicity. Additionally, the American Indian/Alaska Native population is small, and therefore population estimates are unstable and coverage may be overestimated. See DHS for additional information on the limitations of demographic information for immunizations.

Going forward, we will be including this chart of vaccination by race/ethnicity among those eligible in our weekly snapshots so that everyone has a better understanding of the true disparities in vaccination.

What we are doing to address disparities in vaccine coverage

People of color have experienced higher rates of COVID-19 in Dane County but currently have lower vaccination rates compared to people who are white. We have developed a mobile vaccination team in order to assure equitable access to vaccine by prioritizing populations with the greatest access barriers. Our approach includes engaging in education to provide clear and accurate vaccine information, providing mobile clinics to needed areas such as temporary and transitional housing facilities, and coordinating efforts with other local vaccinators, such as African American Health Network, Benevolent Specialties Clinic, and health system partners, to ensure availability of mobile clinics to low income people and people of color in community centers and churches. Vaccinators collectively have conducted more than 40 clinics in the past month and we expect the pace to increase in May. You can read more about these efforts in our news release.