THL study: Vaccine protection against severe COVID-19 disease among health care workers remained excellent for six months after the 2nd dose
Vaccine protection against severe coronavirus disease remained excellent among health care workers (HCW) for six months after the second dose of the vaccine. However, vaccine protection against infection weakened over time. This study, conducted by the Finnish Institute for Health and Welfare (THL), provides important information on the protective effect of vaccination in working-age people.
In Finland vaccination of HCWs started in December 2020. As they were the first group eligible for vaccination, vaccine protection in HCWs can be followed over a longer time period than in the rest of the population. The time interval between the HCWs’ first and second dose of the vaccine was initially 3-4 weeks, but it was later changed to 12 weeks.
The vaccines under study were the two mRNA vaccines – Biontech-Pfizer's Comirnaty and Moderna's Spikevax – and AstraZeneca's adenovirus vector vaccine Vaxzevria. The study also evaluated the protection provided by mixed vaccine series.
The vaccine protection among HCWs aged 16 to 69 years was evaluated between December 2020 and October 2021. The study included more than 400,000 registered HCWs. The vaccine protection was estimated three and six months after the second dose.
The results of the study are preliminary, and they have not yet been subjected to scientific peer review.
No significant differences in duration of protective effect among vaccines of different manufacturers
In the study it was observed that the protective effect against coronavirus infection weakened 3-6 months after the second dose. The protection was at its highest, about 85 percent, during the first three months, decreased to about 65 percent during the next three months. The protective effect of the vaccines against serious disease remained excellent for more than six months after the second dose.
“The weakening of vaccine protection against infection has been observed in other studies as well. In our study we found no significant differences in protection between vaccine series of different manufacturers, and the weakening appeared to be similar across all series”, says Eero Poukka, Medical Specialist at THL.
In this study, the effect of the interval between doses on the vaccine protection was not analysed, but THL experts are currently evaluating the subject.
“In a newly published Canadian study, the longer interval between doses was seen to give 5-10 percent better protection than a short one”, Poukka continues.
THL continues research on effectiveness of coronavirus vaccines
THL constantly monitors the protection given by coronavirus vaccines in different groups, such as the elderly, chronically ill, and working-age adults. Information gleaned from research makes it possible to tailor the Finnish vaccination programme.
“This autumn and in future we will focus on monitoring the vaccine protection against severe outcomes as the prevention of hospitalisations and death is of major national importance.”, says statistician Ulrike Baum.
Further information:
Eero Poukka
Medical Specialist
THL
[email protected]
Ulrike Baum (in English)
Statistician
THL
[email protected]
