This is a summary, written by members of the CITF Secretariat, of:

Nantel S, Bourdin B, Adams K, Carbonneau J, Rabezanaha H, Hamelin ME, McCormack D, Savard P, Longtin Y, Cheng M, DeSerres G, Corbeil J, Gilca V, Baz M, Boivin G, Quach C, Decaluwe H. Immunogenicity of Pfizer-BioNTech COVID-19 mRNA Primary Vaccination Series in Recovered Individuals Depends on Symptoms at Initial Infection. medRxiv. 2022 Jan 1. Doi: 10.1101/2022.03.29.22272714.

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

New research stemming from the CITF-funded RECOVER study indicates that a two-dose primary series of vaccination is paramount for protection against COVID-19, even for those who were previously infected. Those with either no or mild symptoms, as well as anyone who happened not to generate antibodies after infection, did not mount as strong an immune response after one dose compared to individuals who experienced more intense symptoms and/or were seropositive for antibodies after recovery. Despite this, all participants, irrespective of whether they had been previously infected, elicited a robust immune response after a second dose of vaccine. The findings have been released in pre-print, therefore not peer reviewed. The study is led by Drs. Caroline Quach and Hélène Decaluwe from the Sainte-Justine University Hospital and Research Centre.

Key findings:

  • Individuals previously infected with SARS-CoV-2 received a significant boost to their humoral and cellular immune responses following one vaccine dose. This entailed increased levels of antibodies targeting the receptor binding domain (RBD), antibody neutralization capacity, and markers of cell-mediated immunity (specifically, interferon gamma secretion by T-cells).
  • In contrast, uninfected individuals mounted lower levels of the above measures of immunity after one dose. In fact, only one individual in this group had detectable neutralizing antibodies.
  • All participants elicited a robust immune response following two doses of vaccine (median dosing interval of about 16 weeks), further demonstrating the importance of completing the two-dose primary series irrespective of previous infection.
  • The strength of the immune response following one dose is proportional to symptom severity at infection. Among recovered individuals, 69% of those who were asymptomatic mounted high RBD antibody levels, neutralizing capacity, and cell-mediated immunity after one dose compared to most (92%) – but not all – symptomatic individuals.
  • Those who had detectable infection-acquired antibodies six months after infection had a strong general immune response after the first dose. Comparatively, people who recovered from COVID-19 but did not have detectable infection-acquired antibodies after six months generated lower levels of RBD antibodies, neutralizing capacity, and cell-mediated immunity after one dose.

A total of 55 unvaccinated healthcare workers with a previous PCR-confirmed SARS-CoV-2 infection were selected for this study based on symptomology at infection and serostatus six months after infection. A comparator group consisted of 14 unvaccinated and not previously infected healthcare workers. All individuals were vaccinated with Pfizer-BioNTech’s Comirnaty vaccine.