Coronavirus latest: Ohio reports highest number of cases since July

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.

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.

Schools in Greece will reopen on Sept. 14, the government announced Tuesday.

Masks will be mandated indoors. Students in third grade and younger will be taught how to wear their masks properly.

Greece has 10,317 COVID-19 cases and 266 deaths, according to Johns Hopkins University data.

"The pandemic's evolution is dynamic," Greek Education Minister Niki Kerameus told reporters in Athens on Tuesday, adding that measures will be adjusted as necessary on local, regional and national levels.

"We will not hesitate to adopt measures that are more austere depending on the progression of the pandemic," Kerameus said.

ABC News' Christine Theodorou contributed to this report.

The start of in-person learning in New York City public schools has been delayed to Sept. 21, Mayor Bill de Blasio announced Tuesday.

The district's blended-learning plan was scheduled to begin on Sept. 10.

"Teachers who usually get two days of professional development at the beginning of the school year will now get nine," schools Chancellor Richard Carranza said. "We've heard from everyone in our schools that have said we need some more time."

In averting a strike authorization vote by the United Federation of Teachers later Tuesday, de Blasio agreed to delay the start of school to Sept. 16 for all-remote instruction. In-person learning, which is limited to a few days per week, will begin Sept. 21.

The mayor also announced expanded testing for teachers, a significant concession to the United Federation of Teachers.

ABC News' Aaron Katersky contributed to this report.