Intentionally unvaccinated people make up substantial proportion of US measles cases

A review study reveals that unvaccinated people make up a big proportion of US measles cases and whooping cough outbreaks, which also increase the risk among fully vaccinated individuals.

This is the result of a study published today in JAMA. Recent outbreaks of preventable diseases mean that health officials must now pay greater attention to growing trend of vaccine refusal and hesitancy. Saad B. Omer, of Emory University, Atlanta and colleagues examined the association between refusal or exemption and the epidemiology of measles and whooping cough, two vaccine-preventable diseases with recent U.S. outbreaks.

The researchers identified 18 published measles studies, which described 1,416 measles cases (individual age range, 2 weeks-84 years; 178 cases younger than 12 months) and more than half (57 percent) had no history of measles vaccination. Of the 970 measles cases with detailed vaccination data, 574 cases were unvaccinated despite being vaccine eligible and 71 percent of these had nonmedical exemptions (e.g., for religious or philosophical reasons, as opposed to medical contraindications; 42 percent of total).

Among 32 reports of pertussis outbreaks, which included 10,609 individuals for whom vaccination status was reported (age range, 10 days-87 years), the 5 largest statewide epidemics had substantial proportions (range, 24 percent-45 percent) of unvaccinated or under-vaccinated individuals. However, several pertussis outbreaks also occurred in highly vaccinated populations, indicating waning immunity. Nine reports (describing 12 outbreaks) provided detailed vaccination data on unimmunized cases; among 8 of these outbreaks, from 59 percent through 93 percent of unvaccinated individuals were intentionally unvaccinated.

We spoke with Saad B. Omer, one of the authors of the study. He can also be found on Twitter.

RG: What were the results of your study?

Omer: We looked at the role of vaccine refusal in the epidemiology of two important vaccine preventable diseases: measles and whooping cough. For measles, we found that a substantial proportion of cases were unvaccinated and a substantial proportion of those who were unvaccinated had refused due to philosophical or religious reasons.

For several measles outbreaks we had epidemic curves available and since they all varied in time we created a super epidemic curve. This curve was anchored on day zero and looked at the proportion of unvaccinated and vaccinated people. We found that at the beginning of these outbreaks the proportion of unvaccinated people was much higher than in later parts. Basically, unvaccinated people are providing this critical mass of susceptible candidates who start the outbreaks, which through time also progresses amongst the vaccinated. And a huge chunk of the unvaccinated are people who have refused vaccination. They are the tinder that starts the fire.

For whooping cough, even though waning immunity and suboptimal vaccine effectiveness are an issue, we wanted to see if vaccine refusal is still playing a role. We found yes, it is absolutely playing a role.

RG: How does vaccine refusal among segments of the population elevate the risk for those who are vaccinated?

Omer: No vaccine is 100 percent effective. In any other field if something works 80 percent of the time that’s a great intervention. But if vaccines work 80 percent of the time, this is only theoretical because measles and whooping cough vaccines are better than this, there is still a 1 in 5 chance that your kid will be unprotected. The child’s risk would then depend on who they interact with at school, places of worship, extended family and playgrounds etc. This is why vaccines are important because it affects everyone. If you are vaccinated your probability of getting the disease goes down but for most vaccines, it doesn’t become zero.

Then there are people who can’t get vaccinated because they are either too young or they have a medical reason. Some of these people are the most vulnerable, kids on chemotherapy for example. This is why it is all the more important for everyone to get vaccinated and for us to keep the rate of vaccination high.

RG: What are the implications of your study for vaccine practice and policy? Do you have recommendations?

Omer: We wanted to highlight that vaccine refusal plays a role in the epidemiology of two of the more important vaccine preventable diseases. Any intervention policy or any policy to control these diseases should absolutely take into account strategies to address vaccine refusal or hesitancy.

We often apply the best possible science to vaccine development but not to vaccine acceptance. Since it’s equally as important in dictating how many people get the disease, we should apply the same scientific rigor to vaccine acceptance science as we do to vaccine development science.

RG: What is your opinion on people advocating against the use of vaccines or researchers like Andrew Wakefield who falsely linked vaccines to autism?

Omer: Most advocates who work with autism are not anti-vaccine, it’s only a small section of people, most mainstream groups are pro-vaccine. But it’s not just a single paper. When vaccines are successful you don’t see the disease. There is this concept called “availability heuristic”. If you don’t see the disease but you do see and hear about these adverse events happening as a result of vaccines, there is this change in mental calculus. Therefore, in this milieu you have all of these responses.

RG: What is your opinion on making vaccinations mandatory in the US? Or incentivizing them?

Omer: I have a middle of the road approach. I think mandatory vaccinations are a very useful tool. But in most US states mandatory vaccines come with a series of exemptions. This serves a little like a pressure valve. It doesn’t take away the choice completely, but it says that if you want to have your child exempt you have to go through certain procedures. The problem is that, in some states, these procedures are very easy. You just go to a website print off a form, check off a box, and file for an exemption. If you have a mandate that is too absolute, it may push people away but on the other hand if you have a mandate that comes with such a broad and easy series of exemptions you have a problem.


Image courtesy of Pan American Health Organization-PAHO / World Health Organization-WHO.