Original antigenic sin

The short version: original antigenic sin is real, but its clinical impact depends on how much the second virus differs from the first, and repeated vaccination can blunt your response to new strains.

Original antigenic sin means your immune system locks onto the first version of a virus it ever saw. When a drifted variant shows up, your body preferentially boosts antibodies to the old strain instead of building fresh ones against the new one. This was shown clearly in a 1980 experiment: a second myoglobin had to differ by less than about 33% in sequence to trigger a new antibody response; beyond that, the immune system barely reacted [3]. The same pattern appears in human bocavirus infections, where children infected with HBoV1 after prior exposure to HBoV2 or HBoV3 showed weak or absent type-specific IgG responses, consistent with original antigenic sin [5].

The practical consequence for influenza vaccination is that repeated annual shots can reduce your ability to respond to new strains. A 2020 pilot study of 20 adults found that people who had received 3-5 flu vaccines in the prior 5 years had significantly lower antibody-secreting cell responses and lower hemagglutination inhibition antibody fold changes after vaccination compared to those who had received 0-2 shots [2]. For H1N1, the minimally vaccinated group had about 3-fold higher HAI fold change at day 29 (p=0.0005) [2]. The repeatedly vaccinated group started with higher baseline titers, but their ability to mount a fresh response was impaired. No imprinting effect by birth cohort was detected in this small study, but the sample size of 10 per group was too small to rule it out [2].

The evidence here does not include any long-term safety or efficacy data comparing vaccinated to unvaccinated populations. The 2020 study was funded by a manufacturer (GSK) and used surrogate endpoints (antibody titers and antibody-secreting cells), not hard clinical outcomes like hospitalization or death [2]. The 1998 influenza study [1] and the 1980 myoglobin study [3] are mechanistic, not clinical. The HIV primate study [4] found no evidence of original antigenic sin, but that was in a different virus system with a different vaccine platform.

My call: original antigenic sin is a well-documented immunological phenomenon, and the evidence here shows that repeated influenza vaccination can blunt your antibody response to drifted strains. The burden of proof is on the system that recommends annual boosting to show this does not translate into worse clinical outcomes. Confidence: moderate for the blunting effect on antibody surrogates; low for whether that translates into more infections or hospitalizations, because the relevant studies comparing vaccinated to unvaccinated over multiple seasons have not been done.