Months after initial vaccine rollout, many are wondering when McDonough County, and the country as a whole, will reach the “finish line” of the COVID-19 pandemic.  Of course the ultimate goal would be entire eradication of COVID-19, but barring that, epidemiologists are hoping to at least reach “herd immunity”.  Achieving herd immunity, or “population immunity” will be needed in order to return to a “post-COVID” world.

What is Herd Immunity?

Herd immunity refers to protection from a disease for the overall population instead of the individual level.  This means that even if not everyone in the “herd” is immune, there are enough people immune that the vulnerable segment of the population is unlikely to be exposed to the disease.  For a clearer example, refer to the following diagram:

The first picture represents normal disease transmission with no immune individuals.  The person in the middle transmitted the disease to five surrounding contacts, with each one of those contacts transmitting to at least one or two of their own contacts.  This is exactly how COVID-19 spread when it first emerged, because no one had immunity.  Diseases transmit at various rates of contagiousness, and for COVID-19 it is estimated that for every one person infected, they infect anywhere from 1 to 4 other people on average.  The term epidemiologists use for this rate of contagion is R0, or “R Naught”.

The second picture represents a group that has herd immunity.  The infected person in the middle was able to spread the disease to one contact that was not immune, but not the rest of their immediate contacts because they were immune.  Note that not everyone in the group was immune; they can represent children too young for vaccination, immunocompromised individuals, or other vulnerable populations.  Herd immunity does not need everyone immune for it to work, as long as there are enough people that each infected person is unlikely to spread the disease in large amounts.

This can be achieved, and has been for a number of diseases with available vaccines.  Mumps and Polio, for instance, are diseases that are rather rare in the United States because childhood vaccination reduces the amount of possible diseases spreaders, or “vectors”.  These diseases are sometimes described as “eradicated”, but they do exist in areas with low immunity.  Some diseases, such as measles, had a relative herd immunity, but have seen a resurgence of outbreaks because of reduced participation in herd immunity strategies, i.e. vaccination.

How Many Must be Immune to Achieve Herd Immunity?

COVID-19 is a new disease, so there are still unknowns, but answers can be deduced from what we know about epidemiology and disease spread.  Typically a disease will not see a reduction in cases until at least 50% of the population is immune, but for particularly contagious diseases (with high R0) that number may be has high as 90%.  Scientists currently estimate that we will need 70% of the population to be immune to COVID-19 in order to reduce rates without restrictive measures, such as business lockdowns or mask wearing.  For a comparison, as of the time of writing this in April 2021, only about 30% of McDonough County is vaccinated. We have a long way to go!

How Can I help?

There are some helpful ways to help the county build herd immunity, but there are also some not-so-helpful myths.  First, it is important to understand that herd immunity cannot be built by purposefully catching or spreading COVID-19.  Immunity due to contracting COVID-19 is not permanent, and there are known cases of people becoming sick with COVID-19 multiple times.  Not only is this method ineffective, but also dangerous, as it will continue the spread of COVID-19 into vulnerable populations that may end up losing their lives to the illness.

The safest and most effective way to build herd immunity is through mass vaccination.  As explained in other blog posts, vaccination is safe and available to anyone 16 and older in the county.  Vaccines are free through the McDonough County Health Department, even without insurance, and many of the local pharmacies provide appointments as well.  If you have already been vaccinated, you can help by informing others and help them access vaccine clinics hosted in the community.  Even if you are vaccinated, it is still helpful to continue wearing masks in indoor situations and avoiding large crowds to help reduce the chain of spread.  If you are not vaccinated, check with your doctor if you are eligible, and continue to social distance and mask as the CDC and IDPH have recommended since the start of the pandemic.

Stay safe, McDonough County!


John Hopkins School of Public Health

World Health Organization