(Reuters) – The weekly number of new COVID-19 cases in the United States rose last week for the first time after falling for eight straight weeks, an increase that health experts attributed to schools reopening and parties over the Labor Day holiday.

New cases rose 17% to about 287,000 for the week ended Sept. 20, while deaths rose 5.5% to about 5,400 people after falling for the previous four weeks, according to a Reuters analysis of state and county reports.

Thirteen states have seen weekly infections rise for at least two weeks, up from nine states the previous week, according to the Reuters tally. In Arizona, new cases doubled last week.

On average, more than 776 people a day died from COVID-19 last week, with deaths rising in Arkansas, Kansas and Virginia.

After weeks of declining test rates, an average of 812,000 people a day were tested last week. The country set a record of testing over 1 million people on Saturday.

Nationally, the share of all tests that came back positive for COVID-19 fell for a seventh week to 5.0%, well below a recent peak of nearly 9% in mid-July, according to data from The COVID Tracking Project, a volunteer-run effort to track the outbreak.

However, 26 of the 50 states still have positive test rates above the 5% level that the World Health Organization considers concerning. The highest positive test rates are in the Midwest at over 16% in Idaho, Wisconsin, Iowa and South Dakota.

(Writing by Lisa Shumaker; Graphic by Chris Canipe; Editing by Tiffany Wu)