The United States saw a 27 percent increase in new cases of COVID-19 in the week ended July 5 compared to the previous seven days, with 24 states reporting positivity test rates above the level that the World Health Organization flagged as concerning.

Rising coronavirus cases in 39 U.S. states cast a shadow over the nation’s Fourth of July celebrations as health experts worried that holiday parties will cause a further spike in infections that could overwhelm hospitals.

New U.S. COVID-19 cases rose by nearly 50,000 on Wednesday, according to a Reuters tally, marking the biggest one-day spike since the start of the pandemic.