One COVID vaccine dose yields good protection in elderly, 2 studies find

UK COVID-19 vaccination center
UK COVID-19 vaccination center

HM Treasury / Flickr cc

As COVID-19 vaccines were first being approved for emergency use in the United Kingdom, the UK Joint Committee on Vaccination and Immunisation decided to extend the interval before the second dose from 4 to 12 weeks to maximize the amount of people who could be vaccinated. This week, two Lancet Infectious Diseases studies estimated vaccine effectiveness (VE) for the elderly after receiving just one dose.

A study involving more than 10,000 people 65 years and older found a VE of 65% to 68% 5 to 7 weeks post-vaccination, while one involving 466 people 80 years and older found a VE from 71% to 80%.

"In both studies, the confidence intervals for estimates of vaccine efficacy are wide," write Maria Prendecki, MBBS, PhD, and Michelle Willicombe, MD, MBBS, in a commentary. "However, these results provide reassurance that—in older adults (some of whom were frail and had many comorbidities)—both ChAdOx1 nCoV-19 [the AstraZeneca/Oxford vaccine] and BNT162b2 [the Pfizer/BioNTech vaccine] provide protection from COVID-19 and, in cases of breakthrough infection, probably decrease the likelihood of viral transmission."

They add, "Results from at least 42 days after vaccination will be interesting, given the UK strategy of delaying the second dose; however, these findings will be of less direct relevance for older adults in the UK given that more than 90% of people older than 65 years have now received two doses."

As high as 68% protection in those 65 and up

The larger UK study involved 10,412 long-term care facility residents from 310 sites, all over 65 years of age. The median age was 86, 69.6% were women, and 11.1% had previous COVID-19 infection. About three fourths of the group was 80 or older.

During the study period, from Dec 8, 2020, to Mar 15, 2021, 88.0% received at least one COVID-19 vaccine dose, with 67.0% (6,138) receiving AstraZeneca and 33.0% (3,022) receiving Pfizer. Overall, the researchers found 1,335 positive polymerase chain reaction (PCR) results out of 36,352 tests and 670,628 person-days. Of the positive patients, 713 were unvaccinated and 612 were.

Cox regression analysis showed no significant reduction in infection risk until 28 to 34 days post-vaccination, the researchers say. At that point, compared with the unvaccinated residents, those who were vaccinated had an adjusted hazard ratio (aHR) of 0.44 (95% confidence interval [CI], 0.24 to 0.81) and an aHR of 0.38 (95% CI, 0.18 to 0.77) at 35 to 48 days post-vaccination. VE from 35 to 48 days post-vaccination was 68% with the AstraZeneca vaccine and 65% with the Pfizer vaccine.

The researchers also found that the average PCR cycle threshold value was higher for COVID-19 infections that developed at least 28 days post-vaccination (31.3 vs 26.6).

Patients with previous COVID-19 had just as much or more protection against SARS-CoV-2 infection, the researchers note.

The researchers suggest that Pfizer's protective effects may extend beyond 7 weeks but did not say the same for AstraZeneca. Regardless, they conclude that the risk of infection is not eliminated with either vaccine, emphasizing the need for non-pharmaceutical interventions in long-term care facilities.

Higher efficacy in hospitalized elderly

The other UK study followed patients 80 years or older who had received one COVID-19 vaccine dose at least 14 days prior and who were hospitalized for respiratory symptoms in two National Health Service trusts in Bristol.

From Dec 18, 2020, to Feb 26, 2021, 18 of 135 with COVID-19 infections (13.3%) and 90 of 269 non-COVID patients (33.5%) had one dose of the Pfizer vaccine, suggesting 71.4% adjusted VE (95% CI, 46.5 to 90.6). In the same time period, 9 of 36 COVID-19 patients (25.0%) and 53 of 90 non-COVID patients (58.9%) had one dose of the AstraZeneca vaccine, which translates to an estimated adjusted VE of 80.4% (95% CI, 36.4 to 94.5).

This may seem like Pfizer is less effective than AstraZeneca, but the researchers found that restricting the Pfizer analysis to the same time period as AstraZeneca's emergency-use approval increased Pfizer VE to 79.3% (95% CI, 47.0 to 92.5).

While the researchers acknowledge that most cases were likely caused by the Alpha variant (B117), they write, "The findings of our case-control analysis will help to guide strategy development for the use of BNT162b2 and ChAdOx1 nCOV-19 vaccines in clinical practice and should reassure policy makers of the high value of deploying these vaccines, and the importance of administering two doses, in high-risk populations in whom incidence of severe disease and death from SARS-CoV-2 infection remains high."

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