Characteristics of the study population
Pooling together the data from the six participating study-sites, the distribution of study participants by vaccination status in each observation period is shown in Figure 1. Throughout the study, the proportion of unvaccinated individuals remained low, at 3–5% of 65–79-years and 1–2% of ≥80-years. The proportion of vaccinated with the first booster increased progressively in both age groups until the respective second booster recommendation and declined afterwards. In the last observation period (October-November 2022), 25% of ≥ 80-years had been vaccinated with the third booster.
The number of observed COVID-19 hospitalisations ranged between 816 in the first observation period (October-November 2021) and 471 in the last observation period (October-November 2022) among 65–79-years, and between 690 and 665 among ≥80-years. The number of COVID-19-related deaths registered among 65–79-years ranged between 523 in March-April 2022 and 231 in October-November 2022 and between 1470 and 549 among the ≥80-years.
Vaccine effectiveness against COVID-19 hospitalisation (vs un vaccinated)
For the ≥80-years, VE of complete primary vaccination against COVID-19 hospitalisation decreased from 66.9% (95%CI: 60.1; 72.6) in October-November 2021 to 36.1% (95%CI: -27.3; 67.9) by October–November 2022 (Table 1, Figure 2). A similar trend was observed in 65-79 years with corresponding estimates of 86.8% (95%CI: 84.5; 88.8) and 31.5% (95%CI: -7.1; 56.2).
VE of the first booster for the ≥80-years peaked at 95.6% (95%CI: 88.0; 98.4) in October–November 2021 declining to 54.6% (95%CI: 29.6; 70.7) by May 2022 with little variation afterwards (Table 2). For the 65–79-years, the first booster VE declined progressively from 95.4% (95%CI: 92.9; 97) in November-December 2021 to 52.1% (95%CI: 20.2; 71.2) in October–November 2022.
The second booster VE for the ≥80-years started at 82.0% (95%CI: 75.9; 87.0) in June–July 2022 and was 83.9% (95%CI: 77.7; 88.4) in October–November 2022. For 65–79-years, VE was 39.3% (95%CI: -3.9; 64.5) in August–September 2022 but increased to 80.6% (95%CI: 67.2; 88.5) in October–November 2022 (Table 3).
The third booster VE estimate against COVID-19 hospitalisation was 82.0% (95%CI: 71.8; 88.5) but was only available for ≥80-years in October–November 2022 in Portugal.