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

Chantal Williams, MSc, Dana Al-Bargash, MSc, Celeste Macalintal, RN, BScN, Rebecca Stuart, MSc, Anu Seth, RN, BScN, Julienne Latham, RN, Leah Gitterman, MHSc, Stephanie Fedsin, NP, Marion Godoy, RN, BScN, Robert Kozak, PhD, Jennifer L Guthrie, PhD, Heidi Wood, PhD, Allison McGeer, MD, Susy Hota, MD, Elizabeth Rea, MD. COVID-19 Outbreak Associated with a SARS-CoV-2 P. 1 Lineage in a Long-Term Care Home after Implementation of a Vaccination Program–Ontario, April-May 2021. Clinical Infectious Diseases. 2021 Jul 8. DOI:

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

An Ontario long-term care home (LTC) suffered a SARS-CoV-2 outbreak attributed to the Gamma variant. Researchers from a CITF-funded study led by Dr. Allison McGeer have published results in Clinical Infectious Diseases revealing that among the residents and staff considered to be part of the outbreak, 77% and 45% respectively, were fully vaccinated. The estimated vaccine effectiveness against any SARS-CoV-2 infection was 53.5% among residents and 66.2% among staff, increasing to 78.6% among residents when considering vaccine effectiveness against severe disease. There were no severely ill staff.

In an Ontario long-term care (LTC) home, with 124 residents and 224 staff members, staff members were offered the Pfizer BNT162b2 vaccine starting in late December 2020, while residents were offered the Moderna mRNA-1273 vaccine on January 4th and February 2nd. Residents were considered fully vaccinated if they received their second vaccine dose more than 14 days before April 10, 2021, while staff were considered fully vaccinated if they received their second dose over 14 days before their first shift at the LTC.

On April 20th, 2021, an outbreak was declared in the facility. Over the next 24 days, 31 residents and 22 staff tested positive for SARS-CoV-2. All SARS CoV-2 PCR-positive specimens were tested for the Gamma variant using a multiplex real-time PCR assay. Three staff members were excluded from the analysis due to timing of infection and suspected community-acquired infection. In total, 31 residents and 19 staff were classified as positive for the Gamma variant and part of this outbreak.

Among the 64 residents and 101 staff located on the two outbreak floors, 49 (77%) residents and 45 (45%) staff were fully vaccinated, and 3 (5%) residents and 16 (16%) staff were partially vaccinated. Partially vaccinated individuals were excluded from the analysis. The estimated vaccine effectiveness (VE) against any SARS-CoV-2 infection was 53.5% (95% CI, 28.2- 69.8%) among residents and 66.2% (95% CI, 2.3-88.3%) among staff. VE against severe illness was 78.6% (95% CI, 47.9-91.2%) among residents. No staff member became severely ill.

Of the residents infected in the outbreak, 16 were participants in a study of vaccine immunogenicity which was assessing antibody-neutralization capacity 17 days after their second vaccine dose. The median 50% neutralizing titer against the Gamma variant isolate was 1:80 for the five residents who developed COVID-19 (range 0 – 1:80) compared to a median of 1:160 (range 1:80 – ≥1:640) for the 11 uninfected residents. The authors point out that these results support other existing evidence that higher antibody titers are associated with protection against disease.

COVID-19 vaccination remains an effective tool to mitigate severe disease outcomes. This study indicates, however, that other disease prevention and control measures should be considered to limit disease transmission, particularly in light of the more infectious variants.