"We cannot simply carry on as we did before," Tedros Adhanom Ghebreyesus said as he opened the World Health Organization's annual assembly in Geneva.
WHO's member states have begun negotiations towards an international agreement aimed to ensure the world is better equipped to prevent or more effectively respond the next time a pandemic hits.
The process is still in the early stages, but the aim is for agreement to be reached in time for the next World Health Assembly, in May 2024.
"The pandemic accord that member states are now negotiating must be a historic agreement to make a paradigm shift in global health security, recognizing that our fates are interwoven," Tedros said at the start of the 10-day gathering.
That was also the message from a long line of other high-level speakers Sunday.
"I hope the current negotiations on pandemic prevention, preparedness and response result in a strong multilateral approach that saves lives," UN chief Antonio Guterres said, speaking via video message.
Timor-Leste President Jose Manuel Ramos-Horta meanwhile pointed out that "every country, big or small, rich or poor, struggled to mount an adequate response to the pandemic".
"This reminds us that we must build our house before the storm, not during the storm."
'Remember the darkness'
The pandemic has wreaked global devastation and officially killed nearly seven million people, with the true figure believed to be closer to 20 million.
Tedros earlier this month declared that the COVID crisis no longer constitutes a global health emergency.
But he stressed Sunday that "COVID-19 is still with us, it still kills, it's still changing and it still demands our attention".
We may have emerged from "a long, dark tunnel", he said, "but this is the moment to look behind us and remember the darkness of the tunnel, and then ... move forward in the light of the many painful lessons it has taught us".
"Chief among those lessons is that we can only face shared threats with a shared response."
If agreed, the pandemic accord will mark only the second legally-binding health treaty since WHO was created 75 years ago.
Tedros highlighted the success of the first one—the Framework Convention on Tobacco Control adopted exactly 20 years ago.
Over that time, he pointed out, smoking prevalence has dropped by one third globally.
"The WHO FCTC is living proof of the power of global agreements to drive a paradigm shift in global health," he said, voicing hope a pandemic accord could do the same.
"This is the moment for us to write a new chapter in global health history, together."