Effectiveness of COVID-19 vaccination in Finland

The data on the page is updated monthly on the first weekday following the 15th day of the month. The data is updated up to the end of the previous month.

Incidence of infections requiring hospitalisation in susceptible population (per month / 100,000 persons) according to age and vaccination protection, 01.01.2023 – 31.03.2023

Incidence of infections requiring hospitalisation in susceptible population (per month / 100,000 persons) by month according to vaccination protection, Age groups combined, 01.01.2023 – 31.03.2023

Incidence of infections requiring hospitalisation in susceptible population (per month / 100,000 persons) according to gender and vaccination protection, Age groups combined, 01.01.2023 – 31.03.2023

Information

Vaccination protection: No vaccination protection means that the person has not been vaccinated or that sufficient time has not elapsed since the first dose (at least 21 days). Partial vaccination protection means that the first dose was taken at least 21 days ago, but the second dose has not yet been taken or sufficient time has not elapsed (at least 7 days). Full vaccine protection without booster means that at least two vaccination doses were taken at least 7 days ago, but the third dose has not yet been taken or sufficient time has not elapsed (at least 7 days). Full vaccine protection and booster means that at least three vaccination doses were taken at least 7 days ago.

Population groups: Population groups are formed according to vaccination protection, age group and gender. The population group of a person can change over time. The statistics are calculated using age groups 12–19, 20–29, …, 70–79 and 80+, but the results are presented by a crude age distribution. Gender-specific data is not presented by age group or month.

Population and case definitions: The data consists of persons in the Population Information System. In the data, a coronavirus infection and treatment events related to it are set to the coronavirus infection's date, which is retrieved from the Finnish National Infectious Diseases Register. The events are linked to the vaccination protection group to which the person belongs on the date of the infection. Hospitalisations are always a part of infections and intensive care cases are always a part of hospitalisations, meaning that there can be maximum one hospitalisation per infection and one intensive care period per hospitalisation. The hospitalisations are based on inpatient care periods in the Care Register for Health Care, for which the care period start day was -7/+14 days from the coronavirus infection date, and the period must include a diagnosis related to a respiratory infection or coronavirus. The intensive care information are based on care periods in the Intensive Care Register, for which the care period started -7/+28 days from the coronavirus infection date, and the period was coded as COVID-19 care. Irrespective of the cause of death, the deaths include those with a temporal link to a coronavirus infection (0–30 days).

Person-time and follow-up period: The person-time measures the time of all persons involved in the data and the follow-up period measures the time of all susceptible persons. In the data, the calculation of time with each person involved begins on 1 March 2020. For each person, the number of days they have belonged to each vaccination protection and age group is calculated on a monthly basis. The present moment or death determines the accumulation of the person-time and follow-up period. In addition, a coronavirus infection will end the accumulation of the follow-up period.

Incidences in different population groups: Incidence refers to the number of cases relative to the size of the population group and the time it was considered. In order to calculate the incidences, the person-time, follow-up period and events of each population group are summed up on a monthly basis. In general, the incidence of an event in a population group is obtained by dividing the number of events in that group by the corresponding person-time. The incidence by a population level can, for example, be used to assess the burden of disease. The incidence among susceptible persons in the population group considered, i.e. the so-called hazard rate / hazard function, is obtained by dividing the number of events in the group by the group's follow-up time. For example, comparing the hazard rates of unvaccinated and vaccinated population groups can be used to assess the effectiveness of vaccination. At the moment, the data only presents the incidences of the susceptible population. The incidences are presented in units of 30 days per 100,000 persons (i.e. per 100,000 person-months, that is, per 3,000,000 person-days).

Age and calendar time standardisation: Since vaccination coverage, time spent in different vaccination protections groups and the prevalence of developing a severe coronavirus disease differ between age groups, combining the cases and person-times / follow-up periods of different age groups as such does not lead to comparable incidences between different vaccination protection groups. To improve comparability, age-group-specific incidences will be combined using the age distribution of the Finnish population in January 2021 (age standardisation). In addition, different age groups spend time in different vaccination protection groups in different months, which confounds comparison over months. Therefore, when presenting an incidence over several calendar months, the incidences are combined using the relative length of the months. If p(k) is age group k's proportion of the population, q(c) is the calendar month c’s proportion of the days of the reference months in total, and I(k, c) is the (unstandardised) incidence of age group k in the month c, then age and month standardised incidence I(tot) is I(tot) = sum_c q(c) x [sum_k p(k) x I(k,c))] where the sum is over the combined age groups k and the months c. The age-standardised incidence corresponds to Finland's population of January 2021 having the age-specific incidences considered. In order to avoid the interference with age-standardised incidence by individual cases of a very small group, the data of the individual age group will be included if the follow-up period / person-time of the population group considered is at least 2000 person-months.

Comments on comparability: The age-standardised incidences of different vaccination protection groups are only comparable from late summer 2021 onwards, as at that time all age groups begin to have persons in the full vaccination protection group. The age-standardised incidences of partial vaccination protection (despite the standardisation of the calendar month) are poorly comparable, as the majority of different age groups have been in a state of partial protection at different times of the year. In early 2021, the incidences of individual age groups are not comparable, as vaccination within the age group has typically began from different risk groups. This data does not contain a breakdown by risk group. Non-age-standardised incidences can be calculated from the data obtained from the open data interface, which includes the follow-up period for population groups and the number of events. The causes of death, typically registered only after several months, are not combined with the data. That is why no information on the actual cause of death related to a coronavirus infection is available.