“Why does my kid need a second dose of this vaccine? Can’t we just check his titers? What are titers anyway?”
When we give a vaccine, we’re stimulating immunity against a specific disease. Each disease has different “correlates of protection” – things that can be tested to see if immunity has developed. Most vaccines cause the production of highly specific IgG antibodies but that’s not the whole story — other immune system components are produced too.
For example, in the case of babies vaccinated with MMR too young, they often don’t produce enough antibodies, but if they get measles, the T cells stimulated by the vaccine do help prevent severe disease and death. When we talk about titers, we are almost always talking about testing for the presence of disease-specific antibodies in the blood, even though that’s just one piece of the immune system.
How do we know if a vaccine works?
In developing vaccines, scientists generally are looking at some correlates of protection, and measuring those repeatedly over time. But we’re also looking at whether a vaccine works. We know that by tracking how often the disease occurs in people who were vaccinated, or in populations where vaccine programs were instituted. We also learn a lot from outbreaks – for example, the measles outbreak of the 1990s taught us that many kids weren’t being vaccinated because of socioeconomic reasons (they didn’t have access to the vaccines, or their doctors couldn’t afford to stock vaccines), which is what prompted the creation of the Vaccines for Children program. We also learned that not everyone responded to the MMR vaccine (90% respond to the first dose), and that a second dose of MMR would help get more of the population protected (a second dose brings coverage up to 95%).
In the case of the ongoing measles outbreak in Clark County, Washington, we’re learning the area has high rates of non-medical exemption — many kids aren’t being immunized because of misinformation or philosophic preferences. This information may drive efforts to limit or eliminate non-medical exemptions. At the time of writing, none of the 53 cases had received two doses of vaccine, and only one had received a single dose — that gives us good information about how well the vaccine is working.
Top 5 reasons that titers are not a replacement for getting vaccines:
- Checking immunity is not straightforward. We know that not all components of immunity can be tested, but even tests for antibodies are not necessarily super accurate. Commercially available tests, like those you would get at a regular lab, are not necessarily the same tests used in vaccine development research facilities, or even university labs.
- We know that some people will test as having low levels of antibodies, but if they were presented with the disease, their body would ramp up production of those antibodies and the person would be protected. (This is part of how vaccines work, by creating immune memories that can be used to more rapidly respond to disease.) Varicella (chicken pox) is like that — if someone has had two doses of vaccine, or experienced chicken pox disease, we consider them immune, and recommend not checking their antibody levels (except in specific situations) because it’s more likely to say they’re not immune when actually they are.
- Just because someone tests with ‘protective’ levels of antibody right now doesn’t mean they will down the road. This is more likely with vaccines whose protection diminishes over time, like mumps or pertussis. We don’t want to keep testing and testing just to see if someone still has protective levels of antibodies.
- Lab tests aren’t cheap. Let’s say your doctor agrees to order the tests, and your insurance covers them. The Medicare 2019 payments for the measles, mumps and rubella antibody tests total $44.80. Commercial insurance payments are generally higher, which means you’re on the hook for more. But let’s say your doctor thinks you should just go ahead and vaccinate, and not check titers first, but you decide to get the lab tests done yourself, with an online lab order service — that’s $120. How much does MMR vaccine cost? Anywhere from $28 through state immunization programs, to $70 in the private sector. The most costly scenario is thus testing, finding low levels of antibody for one or more of the three viruses, and then immunizing.
- Blood draws aren’t fun. Have you ever drawn blood from a 12-month-old? They have chubby arms and tiny veins, but it’s easy compared to drawing blood from a 5-year-old. Five-year-olds are strong. They scream. They fight. You have to hold them still long enough to find a vein and draw the blood, and that’s if you get the vein on the first try and don’t miss because they got a leg loose and kicked you. How many times are you going to test their titers to avoid vaccinating them?
We vaccinate on a schedule that’s designed and tested to provide protection for the long haul. If there’s an outbreak that gives us more information, we change the schedule to again provide the best protection. If we find that we can get good protection with fewer doses, we change the schedule that way too (and yes, this has happened! HPV vaccine went from three doses to two doses for those who are immunized on time).
Titers can be useful in specific situations, but they’re not a replacement for full, on-time immunization.