The Pfizer/BioNTech and AstraZeneca/Oxford COVID-19 vaccines are as effective at preventing symptomatic illness in people with underlying medical conditions as in the rest of the population, finds a real-world study of more than 1 million at-risk UK residents.
In the observational study, published late last week on the khub preprint server, a team led by Public Health England (PHE) researchers mined the electronic medical records of more than 700 general-practice clinics across the country, representing 10% of the population. They also conducted sentinel antibody testing from December 2020 to May 2021.
While the authors noted that advanced age poses the greatest risk of COVID-19–related hospitalization or death, some underlying conditions have been tied to increased risk. Examples include diabetes, neurologic disease, illnesses or treatments that weaken the immune system (eg, blood cancers, HIV, chemotherapy), and chronic heart, kidney, and liver disease.
Reduced antibody responses have also been noted after two vaccine doses in patients with blood cancers and transplant recipients.
Immune response, vaccine effectiveness
Overall, COVID-19 vaccination generated similar antibody responses in both at-risk patients and their healthy peers. Among immunocompromised patients, however, 70% had coronavirus antibodies after one dose, versus 95% among their non-immunosuppressed counterparts. Reduced antibody levels were observed in immunocompromised patients (-68%) and those with chronic respiratory disease (-65%) after one dose.
As in previous studies, the proportion of coronavirus antibody-positive vaccinees after the first dose was initially higher with the Pfizer vaccine, but the proportions converged after 4 weeks.
In the entire cohort, one dose of any vaccine was roughly 60% effective. Two doses of the Pfizer vaccine were 93.3% effective in patients 16 to 64 years, while they were 78.0% effective with the AstraZeneca vaccine. Among patients 65 years and older, the Pfizer vaccine was 86.7% effective, compared with 76.4% with AstraZeneca.
Two doses of the Pfizer vaccine were 89% effective in at-risk patients 65 years and older, versus 80% with the AstraZeneca vaccine. In at-risk patients 16 to 64 years, two doses of the AstraZeneca vaccine were 81% effective (no data were available for this age-group for Pfizer).
Vaccine effectiveness against symptomatic infection was only 4.0% in the immunocompromised group after one dose of either vaccine, but a second dose showed good effectiveness: 73.0% with Pfizer, 74.6% with AstraZeneca.
Big benefits from second dose
The study authors said that while more data are needed, particularly on vaccine effectiveness against severe disease in immunocompromised patients, "protection against hospitalisation and death in risk groups is expected to be greater than protection against symptomatic disease, as has been seen in studies of the general population."
The researchers recommend that immunosuppressed patients talk with their doctors about the best infection-prevention strategy for them and that their household members also be immunized. "It is vital that anyone with an underlying condition gets both doses, especially people with weakened immune systems as they gain so much more benefit from the second dose," coauthor Mary Ramsay, MBBS, head of immunization at PHE, said in a PHE news release.
In regions with high numbers of COVID-19 cases, "there may be a case for reducing the interval between doses in order to maximise coverage," the authors wrote. "However, other studies have suggested that longer dosing intervals result in improved immune response, therefore such a move may be counterproductive."
According to the news release, a modeling analysis that PHE conducted with Cambridge University estimated that the country's COVID-19 vaccination program had prevented 30,300 deaths and 8.2 million infections by Jun 25. Likewise, vaccines had prevented roughly 7,000 hospitalizations in those 65 to 74 years, 18,000 hospitalizations in those 75 to 84, and 21,300 hospitalizations in those 85 and older by Jun 27.