DURING THE COVID outbreak, and later on with measles and possibly flu, experts talked about herd immunity. The basic idea is that a group (the “herd”) can avoid exposure to a disease by ensuring that enough people are immune so that no sustained chains of transmission can be established. This protects an entire population, especially those who are too young or too sick to be vaccinated.

In order to stop the growth in the number of transmissions, we need to ensure that each individual case causes less than one new infection. So, let’s say that one case leads on average to two more infections, but instead we want that number to be less than one. That means at least 50% of the population needs to be immune, so that at most, only one of the two people who might have been infected by an individual will be.

Imagine that you are infected in a completely susceptible population, and you pass on the infection to five other people (i.e. R=5). In order to prevent an outbreak, at least four out of those five people, or 80% of the population in general, should be immune. Put differently, 20% of the population may remain individually susceptible, but the population would still remain protected. But there’s a problem with this calculation. The assumption underlying the calculation for herd immunity is that people are mixing randomly, and that vaccination is distributed equally among the population. But that is not true.

Geographically, the coverage of vaccination is highly variable on the level of counties and even schools. Opinions and sentiments about vaccination can spread in communities, which may in turn lead to polarized communities with respect to vaccination.

And social media messages may make the problem worse. The sad truth is this – as long as there are communities that harbor strong negative views about vaccination, there will be outbreaks of vaccine-preventable diseases in those communities. These outbreaks will happen even if the population as a whole has achieved the vaccination coverage considered sufficient for herd immunity.

When vaccination is not equally distributed but clustered in communities, large outbreaks are possible even when vaccination coverage in the overall population is high.

If negative vaccination sentiments become more popular in the rest of the population as well, we may start to see more sustained transmission chains. Once those chains are sufficiently frequent to connect under-vaccinated communities, we may again be in a situation of endemic diseases.

Herd immunity against measles requires that 90-95% of the entire population are immune, whereas vaccination coverage is measured as the percentage vaccinated of the target population – which only includes people who are eligible for vaccination. This means that to achieve 95% immunity in the population for measles, vaccination coverage needs to be higher than 95%.

More importantly, there is an ethical argument to be made for the goal of 100% vaccination coverage. It sends the right message. Everyone who can get vaccinated, should get vaccinated – not only to protect themselves, but to protect those who cannot, through herd immunity. By Manny Palomar, PhD (EV Mail June 24-30, 2024 issue)