Study authors: Ida Rask Moustsen-Helms, Hanne-Dorthe Emborg, Jens Nielsen, Katrine Finderup Nielsen, Tyra Grove Krause, Kåre Mølbak, Karina Lauenborg Møller, Ann-Sofie Nicole Berthelsen, Palle Valentiner-Branth
This is a pre-print (not yet peer reviewed) of the first study to report vaccine effectiveness specifically for a long-term care facility population. The population-based cohort study assessed vaccine effectiveness (VE) of the BNT162b2 mRNA vaccine (developed by Pfizer/ BioNTech) in all Long-term Care Facilities (LTCF) residents and all Health Care Workers (HCW) in Denmark. This is also the first study to show VE estimates following 2nd dose.
A total of 39,040 LTCF residents and 331,039 HCW were included. Between 27 December 2020 and 18 February 2021, 95.2% and 86.0% of LTCF residents received their first and second dose, and 27.8% and 24.4% HCW received their first and second dose.
VE for confirmed SARS-CoV-2 infection was estimated for both target groups for different time periods following the 1st and 2nd dose of the vaccine. The study used Poisson and Cox regressions to estimate respectively crude and calendar time-adjusted VE for the vaccine with 95% confidence intervals (CI) for vaccinated versus unvaccinated.
During the study period, there was increased focused testing in LTCF in Denmark and a national testing strategy was in place, with HCW offered weekly PCR-testing. The study therefore included almost all cases of SARS-CoV-2.
- Unvaccinated versus vaccinated
- Within 53 days of the vaccination programme, there were 488 LTCF resident and 5,663 HCW positive cases in unvaccinated groups, versus 57 LTCF resident and 52 HCW within the first 7 days after their 2nd dose. This declined to 27 LTCF residents and 10 HCW beyond the 7 days of their 2nd dose.
- 1st dose vaccinated:
- There was no protective effect observed for LTCF residents after their 1st dose. Median interval until 2nd dose was given was 24 days.
- 17% vaccine effectiveness for health care workers, 14 days after their 1st dose.
- 2nd dose vaccinated (0-7 days after 2nd dose):
- 52% vaccine effectiveness for LTCF residents.
- 46% vaccine effectiveness for health care workers.
- 2nd dose vaccinated (beyond 7 days after 2nd dose):
- 64% vaccine effectiveness LTCF residents.
- 90% vaccine effectiveness health care workers.
Study limitations reported by the authors:
A partial lockdown had been initiated prior to the start of the immunisation programme which might explain the higher VE observed in the unadjusted analysis. However, main results were calendar date-adjusted to address possible bias in the VE estimates due to differences in background risk.
There was possible selection bias into early participation in the vaccination programme. Vaccinations were delayed in some facilities due to ongoing Covid-19 outbreaks, which may have inflated the observed vaccine effectiveness.
Social Care COVID Recovery and Resilience project view on the meaning of this study:
- These findings support maintaining a 2-dose schedule in order to reach maximum benefit of the BNT162b2 mRNA (Pfizer / BioNTech) vaccine. Importantly, the study is the first to provide evidence of the protective effect of two doses of the Pfizer / BioNTech vaccine in a real-world, vulnerable population of care home residents. The outcome measured in this study was confirmed infection with SARS-CoV-2, including symptomatic and asymptomatic cases (the latter would have likely been picked up due to an extensive testing programme being in place).
- Findings of no (for residents) or comparatively low protection (for health care workers) from SARS-CoV-2 infection for a period of approximately 3.5 weeks after the first dose suggests that non-pharmaceutical measures to prevent infections from entering care homes remain essential during the roll-out of the vaccination programme.
- The suggestion that there was no protective effect for LTCF residents following their 1st dose is different from the findings of a recent cohort study from Scotland, demonstrating the effect of one dose of the Pfizer / BioNTech vaccine on reducing hospital admissions across all age bands, and another cohort study from Israel which found a high degree of protection from documented infection following one dose of the Pfizer / BioNTech vaccine. However, there are differences between the three studies with respect to included populations (only the Danish study focused on long-term care populations) and outcomes (only the Danish and the Israeli study measured infection, whereas the Scottish study measured hospitalisations due to Covid-19).
- Due to its observational design, this study – like the studies from Scotland and Israel – is at higher risk of bias compared to gold-standard, randomised controlled trials. However, randomised trials of Covid-19 vaccines have systematically excluded older and frailer people, highlighting the important contribution of this cohort study to improve our understanding of the effects of Covid-19 vaccines in this vulnerable population.
The authors further noted that, despite an observation of no or low vaccine effectiveness following one dose, “there was a short interval (median 24 and 25 days) between first and second dose, and therefore it is unlikely that the full impact of a single dose can be extrapolated from our findings”.
“Studies with a longer follow-up between first and second dose are warranted in order to explore potential advantages of a strategy with a postponed second dose”.
Summary by Sian Smith, Maximilian Salcher-Konrad, and Adelina Comas-Herrera, carried out as part of the Social Care COVID Recovery and Resilience Project (funded by the National Institute for Health Research (NIHR) Policy Research Programme (NIHR202333). The views expressed in this summary are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care).