Coronavirus latest: Ohio reports highest number of cases since July

The governor urged residents to be vigilant as Labor Day approaches.

A pandemic of the novel coronavirus has now killed more than 852,000 people worldwide.

Over 25.5 million people across the globe have been diagnosed with COVID-19, the disease caused by the new respiratory virus, according to data compiled by the Center for Systems Science and Engineering at Johns Hopkins University. The criteria for diagnosis -- through clinical means or a lab test -- has varied from country-to-country. Still, the actual numbers are believed to be much higher due to testing shortages, many unreported cases and suspicions that some national governments are hiding or downplaying the scope of their outbreaks.

The United States is the worst-affected country, with more than 6 million diagnosed cases and at least 184,270 deaths.

California has the most cases of any U.S. state, with more than 713,000 people diagnosed, according to Johns Hopkins data. California is followed by Texas and Florida, with over 633,000 cases and over 631,000 cases respectively.

Nearly 170 vaccine candidates for COVID-19 are being tracked by the World Health Organization, six of which are in crucial phase three trials.

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Ohio reports highest number of cases since July

Ohio on Tuesday reported the highest number of coronavirus cases the state had seen since the end of July.

"We believe this is due, in part, to schools and colleges returning to class," Gov. Mike DeWine tweeted.

The governor urged residents to be vigilant as Labor Day approaches.

"In the weeks after July 4th, we averaged close to 1,500 cases per day -- we believe in large part because of Independence Day gatherings," he tweeted. "Please stay safe this weekend."

Hong Kong begins mass COVID-19 testing effort

The government of Hong Kong has launched a mass COVID-19 testing program that is free and voluntary.

The government said 141 testing sites were opened and around 126,000 people were tested on Tuesday.

Over 674,000 people have made online appointments to get tested, the government said.

Hong Kong -- which has a population around 7.5 million -- has 4,823 coronavirus cases as of Tuesday. Ninety people in Hong Kong have died.

ABC News' Christine Theodorou contributed to this report.

Apple, Google unveil 'Exposure Notifications Express' to make it easier to tell people of possible exposure

Apple and Google announced in a joint statement Tuesday they were rolling out "Exposure Notifications Express" to more easily notify users -- who have opted-in -- of possible COVID-19 exposure.

The update will essentially allow iOS and Android users to receive exposure notifications on their phones from public health officials without the need for a custom app.

"As the next step in our work with public health authorities on Exposure Notifications, we are making it easier and faster for them to use the Exposure Notifications System without the need for them to build and maintain an app," Apple and Google said in their statement.

The notification technology relies on bluetooth data and is only prompted by local public health officials who will provide their name, criteria for triggering a notification and materials for users in the case of exposure.

Maryland, Nevada, Virginia and Washington, D.C. will be the first in the U.S. to deploy the new notification system. It is expected to roll out to other states later this fall.

ABC News' Catherine Thorbecke contributed to this report.

Fauci warns governors to not let Labor Day gatherings ruin the nation's progress

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, is warning state leaders to not let Labor Day gatherings ruin the nation’s progress in the fight against coronavirus.

"You remember following the 4th of July, following Memorial Day, when people understandably get out and congregate, we’ve had surges," Fauci said during Vice President Mike Pence’s weekly call with governors, according to audio obtained by ABC News.

"If we can get by the Labor Day weekend with the cases, the hospitalizations and the deaths going down in general throughout the country, we can get a running start as we go into the fall," Fauci said. "The one thing you don't want is to play whack-a-mole as you go into the fall, where you've gotten everything down and then one comes up."

"You can have a lot of fun without necessarily congregating in crowds with no masks, the situations where you can spread the infection," Fauci said. "We can do it … I have a great deal of faith in the American people that they will do that."

Both Pence and the White House's coronavirus response coordinator, Dr. Deborah Birx, asked governors to tell university presidents to isolate COVID-positive students rather than send them out to spread the virus at home or in an off-campus community.

"The majority of students -- even in online campuses -- are staying in their off-campus housing that they have rented," Birx said. "And so it's really important that these students are continuously tested, isolated and cared for, and don't return to their multi-generational households where they could dramatically increase spread, particularly over the Labor Day weekend."

ABC News' Katherine Faulders contributed to this report.