Many countries lack testing capacity, or test only symptomatic cases or the most serious ones.
The United States - the world's hardest-hit country - is gripped by a third wave of the coronavirus.
The country is approaching 13 million confirmed cases with more than 260,000 dead, or more than a fifth of all infections recorded worldwide.
And the number of positive cases continues to climb, with an increase of 11 percent in new infections detected in a week, or more than 1.2 million cases compared to 1.1 million the previous week.
Record hospitalisations and a surging death toll failed to keep Americans from travelling a day before the Thanksgiving holiday on Thursday, raising fears that the unchecked spread under way is a prelude to further contagion during Christmas.
Daily US deaths from COVID-19 surpassed 2,000 for the first time since May on Tuesday and hospitalisations reached a record 88,000 on Wednesday as the country recorded 2.3 million new infections in the past two weeks alone.
Steep surges in cases typically result in a rising death toll weeks later. Coronavirus deaths reached 2,157 on Tuesday - one person every 40 seconds - with another 170,000 people infected, as millions of Americans disregarded official warnings and travelled for Thanksgiving.
The European region including Russia has had 17.1 million confirmed infections for nearly 388,000 deaths, making it the region with the most recorded cases.
In the last week, 1.7 million new cases have been documented in 52 countries of the European region, 10 percent fewer than the preceding week, which shows that the spread of the pandemic there is slowing.
Behind Europe, the regions with the highest numbers of cases are the United States and Canada (13 million, 272,183 deaths), Latin America and the Caribbean (12.6 million, 438,098 deaths), and Asia (12,1 million, 190.108 deaths).
Follow are the Middle East (3.2 million cases, 75,700 deaths), Africa (2.1 million cases, 50,422 deaths) and Oceania (more than 30.000 cases, 941 deaths).