(302) Changing Profiles of Populations Receiving COVID-19 Vaccines - Differing Underlying Risks for Adverse Events

Publication Number: 1292

Background: During the Covid-19 vaccination campaign in Sweden, the underlying health profile of the vaccinated population changed greatly. The baseline risks for adverse events thus successively altered over time.

Objectives: To illustrate changes in factors that affect the underlying risk for experiencing adverse events in patients following the increasing number of doses of Covid-19 vaccines.

Methods: All Swedish citizens 12 years or older as of 12/31/2020 were included in the study. Information on dates of administration of Covid-19 vaccines was accessed from the National Vaccination Register. In addition to age and sex, the following characteristics were extracted from nation-wide health registers: number of dispensed drugs (by active substance) during 100 days before each vaccination and number of out-patient hospital visits during 180 days preceding each vaccination. All doses through 12/31/2022 were included. For non-vaccinated (by the end of December 2022) individuals, the variables were identified during 100 and 180 days, respectively for drugs and hospital visits, before start of the Covid-19 vaccination campaign in Sweden, 12/27/2020.
These characteristics were summarized by sequential number of dose. Proportion of doses of bivalent vaccines was identified for each dose.

Results: A little over nine million inhabitants were included. A total of 7.9 million received at least one dose of Covid-19 vaccine; 1.2 million remained un-vaccinated by the end of 2022.
As expected, for each dose, all factors in the analysis increased: in non-vaccinated the average age was 37.2 years; in those receiving dose 1, 48.6 years, and by the fifth dose, 75.8 years.
Average number of dispensed drugs increased from 1.0 (unvaccinated) and 2.1 to 5.0, (dose 1 and 5), and number of out-patient hospital visits changed from 0.4 (in 10.6% of the unvaccinated) and 0.6 (in 14% of the vaccinees, dose 1) to 1.0 (in 20% of the vaccinees, dose 5).
Bivalent vaccines were administered mainly as doses 4 and 5, i.e. to an older population with more drugs and hospital visits.

Conclusions: We illustrate the differing population profile of recipients of Covid-19 vaccines in Sweden.
These differences are entirely in line with vaccination policy in Sweden. However, the underlying risk for adverse events is therefore different - and subsequently more difficult to establish as due to the vaccines - in those receiving more doses, especially regarding the bivalent vaccines.