A preprint Swedish cohort study (1.6 million people) compares the risk of Covid-19 infection in vaccinated and unvaccinated people throughout the Covid-19 vaccination roll-out in the country from the 4th of January to the 4th of October 2021.
The study is based on government vaccination and health record linkage, which are generally considered reliable, due to high levels of health data capture by the government in Nordic countries.
Analysis of those who had taken the Pfizer vaccine showed that the effectiveness at preventing symptomatic infection waned from initial high levels to 47% in the period 121-180 days after the vaccine to no convincing effect beyond day 211 post-vaccination (23% reduction; 95% confidence interval, -2 to 41, P = 0·07).
This modelled decline in vaccine effectiveness is illustrated in the figure below. Such an effect was observed for all Covid-19 vaccinations used in the country and observed for more severe health outcomes such as prevention of Covid-19 hospitalisation and death.
 Nordström, Peter and Ballin, Marcel and Nordström, Anna, Effectiveness of Covid-19 Vaccination Against Risk of Symptomatic Infection, Hospitalization, and Death Up to 9 Months: A Swedish Total-Population Cohort Study. Available at SSRN: https://ssrn.com/abstract=3949410 or http://dx.doi.org/10.2139/ssrn.3949410