Two cases of viral breakthrough infections despite full protection from Pfizer mRNA vaccination

Simon Collins, HIV i-Base

A paper in the NEJM reports breakthough SARS-CoV-2 infection in two people out of a cohort of 417 people at 19 and 36 days after receiving a second dose of the Pfizer mRNA vaccine.

Both cases were women (age 51 and 65) with evidence of post-vaccine efficacy but who developed COVID-19 symptoms with infections confirmed by PCR. Viral sequencing showed both cases had T95I, del142–144 and D614G mutations with E484K in one: all of likely clinical importance for variants of concern. The full list of mutations showed some overlap with patterns link to the B.1.1.7 (UK) and B.1.526 (NYC) variants, but also with significant differences.

Strong antibody responses were confirmed post vaccination in case one, including to the most clinically significant mutations, but these were not sufficient to prevent breakthrough infection. As pre-vaccination samples were not available, the paper recognises that infection might have occurred between doses, though unlikely.

These cases show the importance of continued surveillance and limiting the risk of exposure and transmission, even when fully protected from vaccination.


Hacisuleyman E et al. Vaccine breakthrough infections with SARS-CoV-2 variants. NEJM. DOI: 10.1056/NEJMoa2105000. (21 April 2021).

Links to other websites are current at date of posting but not maintained.