Question: My almost 80 year old parents experienced none of the typical side effects from either of their Moderna vaccine shots, and are now concerned that their vaccination may not have “worked”. They are both immune compromised and have other ongoing health issues – including low white blood cell counts. Is there any actual relationship between the efficacy of the vaccine and vaccine side effects? And is there any test they can take to confirm whether or not the vaccine worked for them? And does the vaccine work on people with low white blood cell counts?

Answer:  The fact that your older parents didn’t experience any typical side effects is not surprising and does not warrant much concern. First off, while side effects after the jabs are common, they are not universal.  In fact, data from clinical trials shows that side effects are more likely to be experienced among vaccine recipients <65 years old (see Moderna FDA Briefing Document).  Second, clinical trial data show that the Moderna vaccine is efficacious for individuals with high risk medical conditions (again, see FDA Briefing Document).  Third, real-world effectiveness data out of Israel on the other mRNA vaccine, Pfizer, show that “the vaccine effectiveness was similar for adults 70 years of age or older and for younger age groups for the same time period” (e.g. 94%-96%).  These findings indicate that there is *not* a relationship between vaccine effectiveness and vaccine side effects.  

As to your other questions, there is not an easy way for individuals to test their immune response to the vaccine.  CDC and others recommend against using antibody tests for this purpose as such tests are likely to get false negative results (see Q&A of 2/17).  Folks will just have to trust that the vaccine works even in the absence of side effects or testing.  When it comes to individuals with low white blood counts, we are still gathering data about the effectiveness of vaccines in immunocompromised individuals.  The Moderna clinical trial included more than 7,000 individuals with high risk medical conditions (3,677 in vaccine group, 3,629 in placebo group) and the vaccine proved efficacious in this group of high risk individuals.  However, a subset of this group– those who are severely immunocompromised– was too small to evaluate efficacy outcomes with statistical significance. Ultimately, all of us, including those of us who are fully vaccinated, still need to be cautious– e.g. wear masks, keep distance, avoid crowded places, etc.– especially since there’s a lot we still don’t know (e.g. efficacy among immunocompromised individuals; how well vaccination protects against infection and transmission, etc.).  That said, the evidence we currently have is extremely promising and there’s a lot to be hopeful about!