The number of confirmed novel coronavirus cases worldwide broke through 5 million Thursday.

At least 5,001,494 cases, including 328,227 fatalities, have now been registered. Europe has been the hardest hit with nearly 2 million cases and 169,880 deaths, while the United States has more than 1.5 million cases and over 93,000 deaths.

The U.K. has the second-highest death toll with 35,786, followed closely by Italy with 32,330. The statistics represent only a fraction of the exact total of cases with many countries testing only the most serious infections.

China, ground zero of the virus, has registered more than 84,000 cases and 79,310 recoveries. The country's death toll stands at 4,638. The barely changing figures continue to raise questions in and outside China.

The World Health Organization (WHO) on Wednesday reported the largest single-day increase in coronavirus cases.

The WHO said 106,662 virus cases were reported to the U.N. agency Tuesday – the most in a single day since the outbreak erupted in the Chinese city of Wuhan in December.

WHO chief Tedros Adhanom Ghebreyesus said he was "very concerned" about the situation in low- and middle-income nations.

Overall, the virus has spread to 188 countries since it first emerged. Despite the rising number of cases, most who contract the virus suffer mild symptoms before making a recovery.

There was encouraging news on the scientific front Wednesday, as researchers reported progress on a prototype vaccine while another study into whether infection provides immunity against re-exposure showed promising signs.

"We have demonstrated on rhesus macaques that prototype vaccines protect against SARS-CoV-2 infection and that SARS-CoV-2 infections protected against re-exposure," said senior author Dan Barouch of the Beth Israel Deaconess Medical Center in Boston.

Many governments see the development of an effective vaccine as the only surefire way to fully reopen their economies without risking increased death tolls.