March 29, 2021

expert reaction to preprint on COVID infections in care home residents following a single vaccine dose

A preprint, an unpublished non-peer reviewed study, looks at the effectiveness of a single dose of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccine against SARS-CoV-2 infection in care home residents.

Dr Peter English, Retired Consultant in Communicable Disease Control, Former Editor of Vaccines in Practice Magazine, Immediate past Chair of the BMA Public Health Medicine Committee, said:

“The really good news from this study is that it is further evidence that vaccination is likely to have the potential (if we can vaccinate enough people, and if the virus has a low enough R₀) to generate herd immunity, as it seems to prevent infection even in older people. If people are not infected, they cannot be infectious.

“This paper presents findings from an ongoing study. It is likely that data on the effectiveness of a second dose of vaccine will be presented in due course. A second, “booster” dose of vaccine is likely to increase the quality and duration of the immune response, and thereby reduce further infections and the possibility of spread.

“This is a large study of 10,412 care home residents, all over the age of 64 years (≥65 years). A study of this size can be expected to provide robust data.

“All participants were routinely tested, monthly, for Covid-19, so this study would be able to detect asymptomatic infection (depending on the sensitivity of the tests used).

“As we have seen from previous studies, the protection provided by a single dose of vaccine increases being higher (62%) at 35-48 days than earlier (56% at 28-34 days); and this effect is sustained for “at least 7 weeks”. (It is likely to be sustained for considerably longer; this will be the time they were able to observe the effect for.)

“Furthermore, any infections that did arise following vaccination appeared to have a lower viral load – lower amounts of virus present on the swabs, as estimated by the PCR cycle threshold value (the number of times you double multiply the amount of virus in the sample before it becomes detectable). It is likely that people with lower viral loads will be less infectious.

“Because the B.1.1.5 variant was circulating at the time of the study, it can be assumed that the vaccines are effective against this variant.

“Note that the effectiveness data quoted in this study are for infection. Infection of any degree of severity. Other studies have shown that vaccine is more effective at preventing serious disease (hospitalisation, ICU requirement, death) than at preventing mild-to-moderate disease.”

‘Vaccine effectiveness of the first dose of ChAdOx1 nCoV-19 and BNT162b2 against SARS-CoV-2 infection in residents of Long Term Care Facilities (VIVALDI study)’ by Madhumita Shrotri et al was posted to the preprint server medRxiv on Monday 29 March 2021.

Declared interests

None received.