First dose of ChAdOx1, BNT162b2 vaccines may be protective in LTCF residents after 4 weeks

Long-term care facility (LTCF) residents aged ≥65 years appeared to be protected from COVID-19 4–7 weeks after receiving their first dose of the ChAdOx1 nCoV-19 or BNT162b2 vaccine, according to a study presented at ECCMID 2021.

“Our findings suggest that the risk of SARS-CoV-2 infection is substantially reduced from 28 days after the first dose of either vaccine, and this effect is maintained up to at least 7 weeks after vaccination, with similar protection offered by both vaccine types,” said the authors.

“However, even beyond 4 weeks, a single vaccine dose does not eliminate infection risk, highlighting the continued importance of non-pharmaceutical measures to control transmission within LTCFs,” they said.

The prospective VIVALDI study included 10,412 LTCF residents (from 310 facilities in England) aged ≥65 years (median age 86 years, 69.6 percent female) who underwent routine, asymptomatic SARS-CoV-2 testing between December 8, 2020 and March 15, 2021. About 11 percent (n=1,155) had a history of SARS-CoV-2 infection. Eighty-eight percent had received at least one vaccine dose, 67 and 33 percent first doses of the ChAdOx1 and BNT162b2 vaccine, respectively, and 8.6 percent had received their second dose. The median interval between doses was 63 days.

During said timeframe, 36,352 PCR results were obtained over 670,628 person-days. A total of 1,335 PCR-confirmed positive infections were included, 612 and 713 in vaccinated and unvaccinated individuals, respectively.

Compared with unvaccinated individuals, the risk of PCR-positive SARS-CoV-2 infection declined from 28 days after the first vaccine dose with a vaccine effectiveness of 56 and 62 percent at 28–34 and 35–48 days, respectively (adjusted hazard ratio [adjHR], 0.44, 95 percent confidence interval [CI], 0.24–0.81; p=0.0087 [28–34 days] and adjHR, 0.38, 95 percent CI, 0.19–0.77; p=0.0069 [35–48 days]). [ECCMID 2021, abstract 2782-3; Lancet Infect Dis 2021;doi:10.1016/S1473-3099(21)00289-9]

The risk at the other timepoints were numerically but not significantly reduced compared with unvaccinated individuals (adjHR, 0.64; p=0.083 [0–6 days after first dose]; adjHR, 0.83; p=0.404 [7–13 days]; adjHR, 0.96; p=0.866 [14–20 days]; adjHR, 0.92; p=0.762 [21–27 days]; and adjHR, 0.49; p=0.108 [≥49 days]).

The reduced risk at 35–48 days after the first dose was comparable regardless of whether the individuals received the ChAdOx1 (adjHR, 0.32; p=0.0023 vs unvaccinated) or BNT162b2 (adjHR, 0.35; p=0.0038) vaccine, with vaccine effectiveness of 68 and 65 percent, respectively.

“We found evidence to suggest that protective effects remained beyond 7 weeks for BNT162b2 [adjHR, 0.38; p=0.034] but not for ChAdOx1 [adjHR, 0.64; p=0.329],” the authors noted.

There was also an early reduction in infection risk with ChAdOx1 (adjHR, 0.51; p=0.045 [0–6 days] and adjHR, 0.58; p=0.035 [7–13 days]) but not BNT162b2 (adjHR, 0.84; p=0.663 [0–6 days] and adjHR, 1.11; p=0.700 [7–13 days]).

“[However, these early effects] cannot be attributed to protective effects of the vaccine but might be because recently vaccinated LTCFs were less likely to have ongoing outbreaks of infection,” the authors said.

History of SARS-CoV-2 infection was associated with a reduced likelihood of subsequent infection (adjHR, 0.19). Among unvaccinated residents, those with previous SARS-CoV-2 infection had a lower infection risk than those without prior infection (adjHR, 0.12). Excluding never-vaccinated residents from the analysis suggested further effectiveness of the first dose of the vaccine (76 percent at 35–48 days).

PCR cycle threshold (Ct) values were higher for infections that occurred ≥28 days after vaccination compared with pre-vaccination (mean 31.3 vs 26.6; p<0.0001). “[This suggests] that vaccination might reduce onward transmission of SARS-CoV-2 from individuals with breakthrough infections,” the authors said.

They suggested that both vaccines may be effective against the B.1.1.7 variant, given that the analysis was conducted during the second wave of the pandemic in England, when this variant was emerging.

LTCF residents: A key subgroup

“The greatest effects of SARS-CoV-2 have been in residents of LTCFs, who represent a fraction of the population but account for a disproportionate number of SARS-CoV-2-related deaths in many countries,” the authors pointed out. However, this group has been largely excluded in vaccine trials, leading to a dearth of vaccine efficacy data in this population.

Further research is still warranted to assess the effectiveness of the second vaccine dose as well as its impact on transmission, they added. “This knowledge will be critical to inform policy decisions regarding revaccination schedules in this vulnerable population and the disease control measures needed in the short, medium, and long term to protect LTCFs from future waves of SARS-CoV-2 infection,” the authors concluded.