Expert: For Every 1 Measles Case, There Are Expected to Be 12 to 18 More

Measles, a highly contagious viral disease, recently emerged as a significant public health concern in the United States, challenging the progress made through decades of vaccination efforts. Once considered eliminated in the country by 2000, measles is now experiencing a resurgence that highlights the critical importance of maintaining high vaccination rates. As of April 2023, the Centers for Disease Control and Prevention (CDC) reported 712 cases across more than 20 states, with Texas and New Mexico bearing the brunt of the outbreak, accounting for approximately 90% of all cases, according to Crystal Hodge, PharmD, associate professor of pharmacotherapy at The University of North Texas Health Science Center at Fort Worth.

The current outbreak serves as a stark reminder of the disease's potential severity and transmissibility. Before widespread vaccination in the 1960s, measles caused approximately 500,000 annual cases in the United States, leading to 500 deaths, 50,000 hospitalizations, and 1,000 instances of permanent brain damage. Today, with 97% of current cases occurring in unvaccinated individuals, the outbreak underscores the vital role of immunization in protecting public health. The virus's extraordinary contagiousness—with the ability to infect 12 to 18 people per infected individual in a vulnerable population—makes vaccination not just a personal health choice, but a critical community responsibility.

Crystal Hodge discusses R naughts and the measures of how contagious measles is. | Image Credit: weerapat1003 - stock.adobe.com

"For measles, this has one of the highest R naughts that we have ever studied, with an R naught of 12 to 18," Hodge said. "For every 1 person who has an active measles case, they are expected to infect 12 to 18 other people, assuming, again, a vulnerable, naive population, which is one of the reasons why vaccines are so important."

Drug Topics: What is the current landscape of measles in the United States?

Crystal Hodge, PharmD: So I think it's important to think about this in the context of prevaccine versus what we're currently experiencing. The prevaccine era, generally, this is about before the 60s or so, the 1900s. We had about 500,000 cases of measles per year that led to actually about 500 deaths from measles per year, roughly 50,000 hospitalizations, and about 1000 people every year who ended up with permanent brain damage from encephalitis, which is a potential complication for measles. So that's the historical context. Thankfully, we are not at that point right now. Because of vaccines, we actually for a long time, like between 2000 and roughly 2019 [to] 2024, we really only had about 63 cases per year, roughly 4 outbreaks. An outbreak is 3 or more cases in a year. So, we really were able to eliminate measles from the US; in 2000 we actually did eliminate measles from the US. This year what we are seeing, and last year we actually also saw a big outbreak, but this year, we currently have about 712 cases, so nowhere near where we were in the prevaccine era, but still more than what we normally see. These numbers are as of April 11 from the CDC, so about 712 cases, and that's total. So that's across 20+ states. Now, not all 20 states have a current outbreak. There are about 6 or 7 active outbreaks that are currently ongoing, and of those, the highest counts are in Texas and New Mexico. They account for about 90% of the current outbreak or cases, and most of the current case counts are in our kids, unfortunately. About a third of the cases are in kids less than 5 years old, so probably haven't finished the MMR vaccine series. We do have almost 40% that are between 5 and 19 [years.] But even of the cases that we're seeing, the 712 cases, about 97% of them are in people who are either unvaccinated—maybe they're not eligible yet, because I mentioned that there are some that are really young—or they have unknown vaccine status, and of the cases that we've seen, about 12% of them are being hospitalized, and we've had 3 deaths so far. That's where it currently stands.

Drug Topics: Which states are most affected by the current measles outbreak, and what are the reported case numbers?

Hodge: So, this is a little bit hard to tell, because all of the health departments have a different date that they're reporting. Some of them report on Fridays, and some of them report on Tuesdays and Thursdays. So it kind of changes depending on when you [view it,] and that's why you're seeing so many differences in the numbers of what people are reporting is because it depends on when people are when the health departments are reporting to the national registry into the CDC. The current outbreak is largely based in Texas, with some spillover into New Mexico. That counts for about 90% of cases. In Texas alone, and this updated April 15, there have been 561 cases in Texas, and the vast, vast majority of those have been associated with the ongoing outbreak in West Texas. Only about 7 cases so far are not associated with the outbreak in West Texas, and the majority of the cases that are occurring are in those who are unvaccinated or have an unknown vaccination status. About 175 of those are, again, in our kids that are 0 to 4 [years], so really not able to complete the vaccine series, or right around the age that they should be completing the series. We have had 56 people that are been hospitalized in Texas, and we've had 2 kids who are not vaccinated and had no underlying conditions who, unfortunately, did pass away. The second highest outbreak has been in New Mexico, and as of [April] 12, they had 58 cases, and their epidemiology is a little bit different. About half of their cases are actually in adults, and the 1 death that they had was in an unvaccinated adult, so a little bit of a different makeup, but it does seem to be largely related to that outbreak in West Texas; geographically, they're close by each other. We've got some other states with ongoing outbreaks, Kansas [and] Ohio, for example, and then other states are also seeing active outbreaks, but nowhere near the same numbers as what we're seeing in Texas and in New Mexico.

Drug Topics: What are your biggest concerns regarding the recent measles outbreak in Texas?

Hodge: One of the biggest concerns in Texas is that the reason why it was able to become such an outbreak is because of the low vaccination rates in that in that particular community where it was imported, and unfortunately, I think that one of the things that the COVID pandemic highlighted is that there tends to be a lack of trust and goodwill in health care, and so there is concern that there are going to be growing numbers of larger communities that are unvaccinated or have below the threshold of vaccination needed for herd immunity, and that's going to potentially leave people. Vulnerable to ongoing outbreaks, and one of the reasons why it's been difficult to get this current outbreak under control. So we really need to be thinking about herd immunity. The current vaccination rate that's recommended in order to kind of stop measles in its tracks is through the vaccination rates of greater than 95% or higher, and in some communities in West Texas, the vaccination rates were as low as 50%. I have heard some reports that have been a little bit lower. So that's one of the reasons why you have so many people who are vulnerable to measles and one of the reasons why that outbreak has been difficult to contain. Other things that I'm worried about are some of the social consequences from this degradation in societal trust, right? Like we're trusting our communities to be able to protect people who don't have the option to get vaccinated. Not everyone is eligible for the vaccine, and so I have a lot of sympathy for those people. I work with a very large immunocompromised patient population, for example, and they're not eligible for a live vaccine like MMR. Other things to think about, or to at least have on your radar, are that sometimes measles can have very rare but serious [adverse] effects like the like encephalitis, which is a type of brain infection, and sometimes that can occur 7 [to] 10 years later, after the initial infection. So this is not only the acute infection but the potential downstream effects, and something that we're still investigating is something called immune amnesia. So after having an active measles infection, you can wipe out prior immunity that's been built up through other exposures. Now that's not to say that you can't build that back up, but you don't want to get sick, right? So, that's something that we're actively trying to investigate is what is the impact of immune amnesia from a measles infection.

Drug Topics: How does the contagiousness of measles compare to other common infectious diseases?

Hodge: Yeah, this one's interesting, because how do you measure contagiousness? Well, in the public health literature, we have a value called R naught. You may have heard this because there was a lot of discussion around it when COVID first came out, but R naught is really the reproduction number. So it's the average number of people who will contract the disease in a naive population. So we're assuming that everyone is vulnerable. There's no vaccination, something like that. To give you a frame of context for something like tuberculosis, which is known worldwide for causing significant harm mortality, it's in all the history books that are not for that is about 0.5 to 1. So for every one person who is infected, they're expected to infect another person. For the Spanish flu, the 1918 Spanish flu, that was an epidemic the R naught was around 2 to 3, so every one person that came down with the Spanish flu, they would infect about 2 to 3 other people. For smallpox, that was about 5 to 7. For another vaccine preventable infection, like chicken pox that we all probably got before the vaccine came out, this is a vaccine preventable condition, the R naught is about 10 to 12. So for every 1 person that got chicken pox, another 10 to 12 people that were susceptible to chicken pox would come down with chicken pox. For measles, this has got one of the highest R naughts that we have ever studied, with a R naught of 12 to 18. So for every 1 person who has an active measles case, they are expected to infect 12 to 18 other people, assuming, again, a vulnerable, naive population, which is one of the reasons why vaccines are so important. Other things to think about with the contagiousness of measles is that this is an airborne infection. So it can actually hang around in the air for about 2 hours, and it can live on surfaces for about 2 hours. The incubation period is about 2 weeks, and so you could be exposed and not know it for several weeks, and you actually can spread the disease from 4 days before you have the classic rash like symptoms to 4 days after the rash appears. So you've got about 8 days where you are able to infect other people, and about 4 days where you may not even know that you are infected with it. So it's very, very contagious., and to the point where there are some statistics that say that 90% of people who are nonimmune, if they're exposed, will come down with the infection.

Drug Topics: What are the appropriate steps a pharmacist should take if they suspect a patient has measles?

Hodge: Well, first, before you potentially run into that scenario, I would recommend getting vaccinated so that you can protect yourself. So, try to make sure that that you are protected, first and foremost, and then second, it depends on what the situation is. So if someone is calling in and saying, "Hey, I think I've been exposed to measles," or "I think I have measles, what should I do?" Please, please, please, recommend them to get tested, whether that is at an emergency room or an urgent care, but before they show up anywhere, at any health care facility, they need to call and let them know, because, again, this is an airborne transmitted infection. It's highly contagious. There need to be proper precautions put in place, such as a negative pressure broom, appropriate air filters, all sorts of things. So they need to know ahead of time. So if someone's calling in, that's kind of the best scenario. You can tell them, don't show up anywhere without telling them. If the person is already there, and if they're a pharmacist and maybe they're face-to-face with them, try to isolate them as soon as possible. One of the interim recommendations is to go to a closed-off room so you could try to decrease the amount that it's spreading. There's not a whole lot of data behind this. It's just an interim recommendation, but really make sure that you're isolating them. Do not let them sit in waiting rooms and infecting other people. Contact your local health department. This is an immediately notifiable condition. I will say, the Texas Department of Health and Human Services actually do have a template script for triaging measles contacts for prophylaxis, for vaccinations, and whatnot. So there's some great resources with the Texas Health Department that are available online, but then you also want to make sure that you're asking people about their timing, so whether or not their last measles was exposure, if they happen to know within 72 hours, that's the ideal scenario. If it's been within 6 days, you need to know a little bit more about the exposure and about the patient to see whether or not we're talking about potentially using the vaccine or immunoglobulin, for example, but those are some of the things I would think about if you're a pharmacist in running into potential patients with with measles.

READ MORE: Infectious Disease Resource Center

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