It probably seems like a silly question, but can titers help you tell the difference between vaccine induced vs natural immunity?
After all, you should know if you had the disease naturally or if you had a vaccine, right?
Titers for Vaccine vs Natural Immunity
Still, there might be situations in which you need to know if someone has immunity and you want to know if it was vaccine induced or if they earned their immunity naturally.
Unfortunately, you typically can’t, especially as most vaccines mimic having a natural infection.
In a few situations, if a vaccine targets a very specific part of a virus or bacteria, it may be able possible to tell the difference between vaccine-induced and natural immunity though.
The hepatitis B vaccine, for example, is derived from HBsAg particles, so won’t induce antibodies against hepatitis B core antigen or other hepatitis B proteins.
Most other vaccines, like MMR and Varicella, aren’t so specific. Titers might just show that you are immune, although titer tests aren’t always sensitive enough to pick up vaccine-induced immunity. That’s why, expect for a few high risk situations, titer testing isn’t usually recommended.
“…California patients were genotyped; all were measles genotype B3, which has caused a large outbreak recently in the Philippines…”
Measles Outbreak — California, Dec 2014–Feb 2015
It wasn’t a vaccine strain.
For example, during 2011, 222 cases of measles and 17 outbreaks were reported in the United States, with most cases originating from just five countries (France, Italy, Romania, Spain, and Germany). Six different genotypes were identified, including B3, D4, G3, D8, H1, and D9. No vaccine strains…
And no, it doesn’t matter that the vaccine strain of measles, genotype A, differs from all of the wild strains of measles we see in the outbreaks.
“Vaccine induced immunity protects against all virus strains. Measles is considered a monotypic virus despite the genetic variations.”
Factsheet about measles
Unlike the flu, HPV, and pneumococcal bacteria, in which vaccines only protect against different serotypes, in the case of measles, the genotype simply helps us figure out where the measles case came from.
But if it isn’t the vaccine strain, then why do they that is it important to rapidly identify wild strains vs vaccine strains?
“During measles outbreaks, it is important to be able to rapidly distinguish between measles cases and vaccine reactions to avoid unnecessary outbreak response measures such as case isolation and contact investigations.”
Roy et al on Rapid Identification of Measles Virus Vaccine Genotype by Real-Time PCR
That’s easy to answer.
Outbreaks typically trigger a lot of folks to get vaccinated. While that’s great, one possible problem is that some of those folks might develop a fever and/or rash after their MMR vaccine. So it is important to quickly figure out whether they are part of the outbreak and have a wild strain (maybe they were exposed before their vaccine could start to work) or are having a common, mild vaccine reaction.
But couldn’t they have vaccine-associated measles if they have a rash and fever and a vaccine strain? Theoretically, but then they would likely have true measles symptoms. And even in these rare case reports, the children didn’t spread the measles to anyone else.
So why are you waiting to know the genotype of the measles strain causing the outbreak in your area? Hopefully, it isn’t to help you decide whether or not to vaccinate and protect your kids. While it is interesting to know where the outbreak originated, you can bet that it isn’t a vaccine strain.