Italy, Scotland and Belgium also had significant excess deaths, while in some countries such as New Zealand there was no marked change or even — as in the case of Bulgaria — a decrease.
While the authors note that there are differences in the compositions of populations, such as age and the prevalence of pre-existing conditions that contribute to mortality rates, government efforts to suppress transmission of the virus and the ability of national health systems to cope with the pandemic also played a role.
Amitava Banerjee, a professor of clinical data science at University College London who wasn't involved in the study, said it was well designed and had used standardised methods.
He noted that the comparison between death rates in the United Kingdom and New Zealand, where the age of the population and the rates of pre-existing conditions such as obesity are similar, supports the argument that other factors contributed to the differing mortality figures.
"Even if vaccines and better treatments for severe [Covid-19] infection are developed, the way to minimise excess deaths is to reduce the infection rate through population level measures," said Banerjee.
These include lockdowns, protecting high risk groups,and establishing effective "test, trace and isolate" systems, he said.
Germany, which like the United States was not among the 21 countries examined in the study, has seen fewer deaths so far in 2020 than in some recent years, according to the head of the country's disease control agency.
While the reasons for this are complex and may take time to fully understand, a decline in hospital infections and the absence of any reported measles cases in Germany since March indicate that social distancing, mask wearing and hand washing play a role.
"The measures that were introduced because of Covid have further effects, and they're positive, that much is clear," Lothar Wieler, who heads the Robert Koch Institute, told reporters in Berlin.