Vaccine effectiveness against COVID-19 severity in the elderly

A recent study posted to the medRxiv* pre-print server assessed vaccine effectiveness (VE) against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in elderly patients before and after the spread of the SARS-CoV-2 Omicron variant of concern (VOC). 

SARS-CoV-2 vaccines have played an instrumental role in curbing the impact of the coronavirus disease 2019 (COVID-19) pandemic. However, with the emergence of novel SARS-CoV-2 VOCs, VE against infections caused by the VOCs requires extensive research.

About the study

The present study estimated the VE of SARS-CoV-2 vaccines against disease severity, related hospitalizations, and intensive care unit (ICU) admissions in elderly COVID-19 patients in Finland after two and three vaccine doses. The study also assessed the impact of the SARS-CoV-2 Omicron VOC infection on elderly patients.

The study population included persons aged 70 years and more while individual-level demographic data were obtained from the population information system in Finland. Data like age, gender, residence, hospital residency, influenza vaccination, time spent hospitalized from 2015 to 2019, and comorbidities were also collected.

The team evaluated the differences between the three approved vaccine brands, namely Spikevax, Vaxzevria, and Comirnaty, and the number of doses administered of each vaccine. The study also considered the time passed since the vaccination into specific time points, for example, 21 and 84 days post the first dose, 14, 91, and 181 days post the second dose, and 14 and 61 days post the third dose.

The primary outcome of this study was the duration between COVID-19-related hospitalization and a SARS-CoV-2-positive polymerase chain reaction (PCR) test. COVID-19-related hospitalizations were defined as patient admission with the primary diagnosis of COVID-19, acute respiratory tract infections, or severe complications of lower respiratory tract infections. A timely association of hospitalization with COVID-19 infection occurred only if the patient was tested COVID-19-positive 14 days before or seven days after hospital admission.

The secondary outcome was evaluated in terms of COVID-19-related ICU admission which was considered SARS-CoV-2-related if the patient’s physician marked it so, or, if the patient tested COVID-19-positive during the hospital stay. ICU admissions were identified from patient record data obtained from the Finnish intensive care consortium's quality register for intensive care.   

Results

The study results showed that out of the 897,932 individuals eligible for the study, 4% were unvaccinated, 6% were vaccinated with two vaccine doses of Comirnaty, and 66% were vaccinated with three doses of Comirnaty. There was a dosing interval of 84 days between the first and the second dose and 187 days between the second and the third dose.

There were 637 unvaccinated persons among the 1,522 COVID-19 hospitalized patients and 131 unvaccinated individuals among the 234 ICU admissions. Within 14 to 90 days and 91 to 180 days of administration of the second Comirnaty dose, VE of 93% and 87%, respectively, was observed against hospitalization. These values increased to 96% in the 14 to 60 days post third dose vaccination. VE against ICU admission at these time points was higher than that against hospitalization. Also, a VE of 76% against hospitalization was found after two homologous doses of Vaxzevria in the first 91 to 180 days.

A slight decrease in VE as a subject to increasing age and the presence of comorbidities was also observed. In individuals aged 70 to 79 years, the VE of Comirnaty after the second dose was 95% in the first 14 to 90 days and 88% in the 91 to 180 days; whereas the VE at the same time points corresponded to 89% and 83%, respectively, for patients aged 80 to 89 years. The presence of predisposed comorbidities lowered the VE from 92% to 83% in the 91 to 180 days post the administration of the second Comirnaty vaccine dose while a reduction from 99% to 95% was found 14 to 60 days after the third Comirnaty dose.

The VE of Comirnaty against COVID-19-related hospitalization in the fourth quarter of 2021 was 90% in the first 14 to 90 days post-second vaccination which increased to 96% in the 14 to 60 days post third vaccination, while the same in the SARS-CoV-2 Omicron-dominated first quarter of 2022 were 91% and 95%, respectively. While in the fourth quarter, no significant difference was found in the VE 14 to 90 days and 91 to 180 days after the second dose, the VE in the first quarter reduced to 76% in the 91 to 180 days post-second dose.    

Conclusion

Overall, the study findings showed that COVID-19 vaccines have provided sufficient protection against COVID-19-related hospitalization and ICU admission in the high-risk elderly population. The team also found evidence for waning VE across the first six months after vaccination with the two-dose series while the third dose significantly improved the VE for at least two to three months post third dose administration.  

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.