Is the second vaccine dose necessary if seroconversion occurred after the first dose?

Question

My question is applicable to all multi-dose vaccines with vaccination intervals of several weeks, though the specific case that triggered my question was the chickenpox vaccine Varilrix/GSK.

My assumption is that, if after the first dose of the chickenpox vaccine, the patient develops chickenpox-like blisters (more than 10), that seroconversion occurred and that the patient would have detectable and lasting antibodies against chickenpox.

If that is true and if seroconversion certainly happened, is it at all necessary to vaccinate a second time?

My analogy is that, if actual chickenpox infection conveys long lasting immunity and the vaccine-strain infection certainly happened, will that be enough for long-lasting immunity?

Or is vaccine effectiveness independent of how the immune system reacts to a single dose?

Answer 1

The only way to know the the level of conversion is through bloodwork to measure it and even that can be problematic. From the CD site:

Laboratory evidence of immunity or laboratory confirmation of disease

Commercial assays can be used to assess disease-induced immunity, but they lack sensitivity to always detect vaccine-induced immunity

(i.e., they may yield false-negative results).

The issue there is that even the CDC knows that immunity can be undetectible (for what reason I do not know), and yet you are protected. If it's a vaccine induced infection, I think you'd be subject to the same risk of false negatives.

The only way to measure is titer though. In fact, you cannot actually know the current vaccine schedule is leaving you protected. I have a sibling that has received a particular vaccine a number of times (3 times more than what is considered necessary) and regardless, if she has a titer pulled, it shows no immunity (not varicella, we had that as children). We don't typically titer all people because it isn't generally a good use of resources. You can though, request one to be done & certainly after an outbreak of varicella, to simply see if another shot is warranted or not.

The quote provided is about 1/4 of the way down on that link.

https://www.cdc.gov/vaccines/pubs/surv-manual/chpt17-varicella.html