He told a news conference that as transmission increases, governments must also deploy tried-and-tested measures like mask-wearing and improving ventilation.
Countries including New Zealand and Australia have seen a jump in cases and hospitalisations as new Omicron variants that can evade the protection offered by vaccines emerge.
Some Australian health experts are calling on state governments to reintroduce mask rules to combat the spread.
"Sub-variants of Omicron, like BA. 4 and BA. 5, continue to drive waves of cases, hospitalisation and death around the world," Tedros said.
"Surveillance has reduced significantly – including testing and sequencing – making it increasingly difficult to assess the impact of variants on transmission, disease characteristics and the effectiveness of countermeasures."
Furthermore, tests, treatments and vaccines are not being deployed effectively.
"The virus is running freely and countries are not effectively managing the disease burden based on their capacity," he said, in terms of hospitalisation of acute cases and the expanding number of people with long Covid.
'Uncertain and unpredictable'
The WHO's Covid-19 emergency committee met Friday via video conference and determined the pandemic remains a "Public Health Emergency of International Concern" – the highest alarm the organisation can sound.
WHO emergencies director Michael Ryan told the meeting recent changes in testing policies were hindering the detection of cases and the monitoring of virus evolution.
The committee stressed the need to reduce transmission as the implications of a pandemic caused by a new respiratory virus would not be fully understood, the WHO said Monday.
The group voiced concern over steep reductions in testing, resulting in reduced surveillance and genomic sequencing.
"This impedes assessments of circulating and emerging variants of the virus," the WHO said, feeding the inability to interpret trends in transmission.
The committee said the trajectory of virus evolution and the characteristics of emerging variants remained "uncertain and unpredictable".
It said the absence of measures to reduce transmission increased the likelihood of "new, fitter variants emerging, with different degrees of virulence, transmissibility, and immune escape potential".
Booster boost
Meanwhile, the WHO's European office recommended a second booster shot of a Covid vaccine for older people and vulnerable groups.
Covid cases have been rising sharply since the end of May around most of Europe. The call followed the EU's health and medicine agencies recommendation on Monday of a second booster shot for people over 60 years old.
Coronavirus cases have risen 57 per cent in Moscow over the past week, the Russian capital's health authorities said.
"We recommend that you wear a mask in public places because the new Omicron sub-variants BA. 4 and BA. 5 spread more rapidly from person to person," Moscow social services wrote on Telegram.
Hundreds of thousands of people were under lockdown in a small Chinese city after just one case of Covid-19 was detected, as Beijing's strict no-tolerance virus strategy showed no sign of abating.
The steelmaking hub of Wugang in Henan province announced three days of "closed control".
None of the city's 320,000 people are allowed outside their homes until midday tomorrow. Local authorities were to deliver basic necessities.
China is the last major economy glued to a zero-Covid policy, crushing new outbreaks with snap lockdowns, forced quarantines and onerous travel curbs despite mounting public fatigue and damage to the economy.
