Low risk of severe COVID-19 after first dose of Pfizer and AstraZeneca vaccines in Scotland

By Dr. Liji Thomas, MDOct 10 2021Reviewed by Benedette Cuffari, M.Sc.

An encouraging analysis published in journal The Lancet Respiratory Medicine shows that hospital admissions and deaths due to the coronavirus disease 2019 (COVID-19) were drastically reduced at two weeks or more from the first dose of either the Pfizer or AstraZeneca vaccine. However, high-risk groups continued to experience increased rates of severe illness after one dose.

Study: COVID-19 Hospital Admissions and Deaths After BNT162b2 And Chadox1 NCov-19 Vaccinations In 2·57 Million People In Scotland (EAVE II): A Prospective Cohort Study. Image Credit: Marc Bruxelle / Shutterstock.com

Background

The United Kingdom has accounted for over 8.1 million cases of COVID-19 out of the global total of more than 237 million. Over 138,000 people have died in the U.K., despite the strict implementation of non-pharmaceutical measures like social distancing, mask use, lockdowns, hand washing, and restrictions on public gatherings.

Undoubtedly, these measures reduced viral transmission but at a high cost to the ordinary quality of life. Vaccination was thus perceived to be the only way back to a semblance of normal life.

COVID-19 vaccination in Scotland began in December 2020, with the deployment of the Pfizer–BioNTech BNT162b2 COVID-19 vaccine. A month later, the Oxford–AstraZeneca ChAdOx1 nCoV-19 vaccine was rolled out. Both vaccines have a two-dose prime-boost regimen.

Cases in Scotland began to peak from the start of December 2020 to a peak on January 4, 2021, which was when the second vaccine was deployed. In view of vaccine shortages, the U.K. Joint Committee on Vaccination and Immunization (JCVI) began to offer a single dose and then a second dose after three months. The aim of the delayed second dose was to extend vaccine coverage to a greater proportion of the susceptible population.

People aged 80 years or above, those in long-term care facilities, health workers, and care workers were prioritized in the first vaccination round, followed by those deemed to be at higher risk on a clinical basis. Eligibility was then progressively extended to younger age groups.

“Although this advice differs from trial evidence and manufacturers guidance on timing between doses, data from mathematical models suggest that this approach has the potential to contain the pandemic quickly, achieving optimal public health benefit, and could result in reduced cumulative mortality under certain conditions.”  

While the Pfizer vaccine prevented 95% of symptomatic COVID-19, the AstraZeneca vaccine showed a real-world efficacy of 70%. In a real-world study, however, the corresponding figures were 91% and 88%, respectively, for hospitalizations at about one month from first-dose vaccination.

Breakthrough infections are still possible, and a fraction of these may become severe. It is necessary to understand how frequent breakthrough infections occur in order to shape clinical practice and public health interventions, including vaccination policies.

The current study assessed COVID-19 hospitalizations and deaths at 14 or more days from the first dose of either vaccine in relation to clinical and demographic factors. Otherwise known as the Early Pandemic Evaluation and Enhanced Surveillance of COVID-19 (EAVE II) platform, this was an open, real-time, and prospective observational study of a national population that utilized data on vaccination, primary health care, laboratory tests, hospitalization, and death from COVID-19.

Study findings

The researchers found that one dose of the Pfizer vaccine was administered to a third of the cohort, with the rest receiving the AstraZeneca vaccine. During the study period, 27% also received the second dose, with approximately 60% getting the Pfizer vaccine.

Of the over 2.5 million individuals in this cohort, about 1,200 were hospitalized or died 14 or more days from the first dose. More specifically, 230 of these were in-hospital deaths.

The event rate of almost 5 for every 1,000 person-years with one dose of the vaccine dropped to less than 1/1,000 person-years after two doses. The number of events was comparable with both vaccines. The differences were mostly in the oldest age group, at 80 years or more, who experienced 63 events per 1,000 person-years after one dose of the Pfizer as compared with 11 with the ChAdOx1 vaccine.

Risk factors for severe outcomes with COVID-19 included age, a greater number of underlying illnesses, history of hospitalization within the month prior to vaccination, working in a place or profession with a high risk of exposure, long-term care facility residence, residence in a more deprived locality, male sex, and ex-smoker history. Those who had been previously infected with the virus were unlikely to have severe outcomes.

The highest risk for severe outcomes occurred with symptomatic infection during the 2-3 weeks after the first dose. Comorbidities including chronic renal disease, asthma, heart failure, type 2 diabetes, heart disease, and dementia increased the risk of hospitalization or death from COVID-19 following vaccination. However, asthma and heart failure were not associated with worse outcomes when the Pfizer vaccine group was separately analyzed.

Implications

The results of the current study show that hospitalizations and deaths among low-risk individuals were low after one dose of either the Pfizer or AstraZeneca vaccine, at less than 0.05% overall. However, high-risk groups continued to show an elevated risk compared to the whole group.

Interestingly, ex-smokers were the only group with a higher risk of severe outcomes in the vaccinated population who were not represented in the unvaccinated high-risk population. The protective effect of immunity associated with a prior episode of COVID-19 was clearly observable. Earlier work corroborates this, showing that one dose of a messenger ribonucleic acid (mRNA) vaccine in a previously infected person is as protective as two doses in a COVID-19-naïve individual.

The rate of hospitalization or death from COVID-19 was 4.6 events per 1000 person-years among the mostly high-risk older population. This was comparable to 8.57 events per 1000 person-years among the unvaccinated, much younger, and low-risk population in Scotland. The magnitude of the reduction in severe disease should be understood in this context, especially with a high incidence of COVID-19 during the study period, but with a lockdown in place.

“We plan to continue analysis as the U.K. relaxes lockdown restrictions, extends the vaccination program to younger and healthier individuals, and introduces Moderna and other vaccines into its national vaccination program.”