Post-vaccination COVID-19: A case-control study and genomic analysis of 119 breakthrough infections in partially vaccinated individuals

Clin Infect Dis. 2021 Aug 19;ciab714. doi: 10.1093/cid/ciab714. Online ahead of print.


Background: Post-vaccination infections challenge the control of the COVID-19 pandemic.

Methods: We matched 119 cases of post-vaccination SARS-CoV-2 infection with BNT162b2 mRNA, or ChAdOx1 nCOV-19, to 476 unvaccinated patients with COVID-19 (Sept 2020-March 2021), according to age and sex. Differences in 60-day all-cause mortality, hospital admission, and hospital length of stay were evaluated. Phylogenetic, single nucleotide polymorphism (SNP) and minority variant allele (MVA) full genome sequencing analysis was performed.

Results: 116/119 cases developed COVID-19 post first vaccination dose (median 14 days, IQR 9 - 24 days). Overall, 13/119 (10∙9%) cases and 158/476 (33∙2%) controls died (p<0.001), corresponding to 4∙5 number needed to treat (NNT). Multivariably, vaccination was associated with 69∙3% (95%CI 45∙8 - 82∙6) relative risk (RR) reduction in mortality. Similar results were seen in subgroup analysis for patients with infection onset ≥14 days after first vaccination (RR reduction 65∙1%, 95%CI 27∙2 - 83∙2, NNT 4∙5), and across vaccine subgroups (BNT162b2: RR reduction 66%, 95%CI 34∙9 - 82∙2, NNT 4∙7, ChAdOx1: RR reduction 78∙4%, 95%CI 30∙4 - 93∙3, NNT 4∙1). Hospital admissions (OR 0∙80, 95%CI 0∙51 - 1∙28), and length of stay (-1∙89 days, 95%CI -4∙57 - 0∙78) were lower for cases, while Ct values were higher (30∙8 versus 28∙8, p = 0.053). B.1.1.7 was the predominant lineage in cases (100/108, 92.6%) and controls (341/446, 76.5%). Genomic analysis identified one post-vaccination case harboring the E484K vaccine escape mutation (B.1.525 lineage).

Conclusions: Previous vaccination reduces mortality when B.1.1.7 is the predominant lineage. No significant lineage-specific genomic changes during phylogenetic, SNP and MVA analysis were detected.

Keywords: COVID-19; Genomics; Mortality; Mutation; Vaccination.