A dozen crew members on a cruise ship docked off the coast of Greece have now tested negative for COVID-19 twice after initially testing positive, according to the ship's operator TUI cruises.

The 12 crew members aboard the Mein Shiff 6 cruise liner tested positive for the virus on Monday and were immediately isolated, along with 24 other crew members who had come into contact with them. But those "unclear positive" results were later identified as negative by two further tests -- PCR (polymerase chain reaction) tests performed on the ship by TUI Cruises and rapid antigen tests conducted by Greek authorities in the port city of Piraeus, according to a press release from TUI Cruises.

"A further PCR test, carried out by Greek authorities, should confirm this again. The results are expected later today," the company said in the statement Tuesday, calling the incident a "possible false alarm."

None of the 12 crew members have shown any symptoms, and rapid antigen tests for the 24 close contacts also came back negative.

The ship departed Sunday night from the port city of Heraklion, the capital of the Greek island of Crete, with all 922 guests testing negative for COVID-19 prior to boarding. Crew members are tested for the virus every two weeks, according to a TUI spokesperson.

The company resumed cruise operations in late July in Germany and started sailing in Greece on Sept. 13, according to the spokesperson.

ABC News' Christine Theodorou contributed to this report.

A spokesperson for the University of Oxford told ABC News Tuesday that volunteers under the age of 18 are currently not being recruited into the clinical trail of an experimental COVID-19 vaccine.

"We have prioritized adults as there is strong evidence that children are much less likely than adults to become unwell with COVID-19," the spokesperson said. "Whilst we do plan to run a trial in this age group, we first hope to generate evidence of the vaccine efficacy in adults, and as such cannot set a date for when the under 18 element of the trial may begin."

The studies to assess the safety and efficacy of the so-called ChAdOx1 nCoV-19 vaccine began in the United Kingdom in April. In addition to the U.K.-based trial, phase III studies have now begun in Brazil, South Africa and the United States to assess the vaccine in other populations, along with a related trial in India.

ABC News' Zoe Magee contributed to this report.

Israel now has the world's highest daily COVID-19 death rate per million people, surpassing that of the United States, according to a report published Tuesday morning by an Israeli military task force.

The report shows that the daily COVID-19 death rate over the past week has been 3.5 per million people in Israel and 2.2 per million people in the United States.

Meanwhile, Israel continues to have the world's highest weekly COVID-19 infection rate per million people, followed by France and the United States, according to the report from the task force, which was formed by the Israel Defense Force's Military Intelligence Directorate and advises the country's health ministry.

The report also shows that the number of COVID-19 patients hospitalized in critical condition has increased by 70% over the last month and is 10 times higher than three months ago.

Since the start of the coronavirus pandemic, Israel has reported more than 233,000 confirmed cases with over 1,500 deaths, according to data from the country's health ministry.

Israel entered its second nationwide coronavirus lockdown on Sept. 18, as the Jewish High Holidays began. The country's health minister, Yuli Edelstein, said Tuesday that there was "no way" the lockdown would be lifted after three weeks as originally planned.

ABC News' Bruno Nota contributed to this report.

Moscow Mayor Sergey Sobyanin announced Tuesday that fall break for all schools will be extended from one week to two, due to rising COVID-19 cases in Russia's capital.

"On the recommendation of sanitary doctors, taking into account the autumn increase in colds and the growth of the number of identified cases of COVID, I decided to prolong the duration of the autumn holidays to two weeks and to hold them at the same time in all schools -- from October 5 to 18," Sobyanin said in a message to parents and students, which was posted on his official website.

There will be no remote learning during the break. Moscow's kindergartens and preschool groups will continue working as usual, according to Sobyanin.

The mayor urged families to use this time as "an opportunity" to reduce the spread of COVID-19. He advised children against visiting shopping malls or riding public transportation "for fun."

"I very much ask parents to explain to their children that it is best to spend holiday time at home or in the countryside," he said. "Today, a significant part of those infected, often asymptomatic, are children. When they come home, they very easily transmit the virus to adults and elderly family members, who suffer much more severely from the illness."

The move comes as Moscow confirmed some 2,300 new cases of COVID-19 in the past 24 hours, the city's highest daily tally since May 31. The daily growth of cases in the capital stands at 0.8%, according to Russia’s coronavirus response headquarters.

ABC News' Alina Lobzina contributed to this report.