TOKYO — Japan on Friday declared states of emergency in Tokyo, Osaka and surrounding areas in an effort to stem a widening coronavirus outbreak three months before the country plans to host the Summer Olympics.

The measures will take effect on Sunday, Prime Minister Yoshihide Suga said, calling them “a short and concentrated measure” to slow the virus’s spread during the Golden Week holiday, traditionally one of the year’s busiest travel periods.

The health effects of Covid-19 not only can stretch for months but appear to increase the risk of death and chronic medical conditions, even in people who were never sick enough to be hospitalized, a large new study finds.

In the study, published Thursday in the journal Nature, researchers looked at medical records of more than 73,000 people across the United States whose coronavirus infections did not require hospitalization. Between one and six months after becoming infected, those patients had a significantly greater risk of death — 60 percent higher — than people who had not been infected with the virus.