Unprecedented quantities of vaccines could be produced by 28 manufacturers in 10 countries over the next two years to tackle the COVID-19 pandemic, the U.N. children's agency UNICEF said on Thursday, as it announced it would help lead efforts to procure and distribute them.

UNICEF's role is part of a COVID-19 vaccine allocation plan - known as COVAX and co-led by the World Health Organization - that aims to buy and provide equitable access to the shots. So far, 76 wealthy nations committed to joining the COVAX effort.

The U.S., however, has opted not to join the global effort, the Trump administration said earlier this week. This, in part because the World Health Organization is involved.

UNICEF said the 28 vaccine manufacturers had shared their annual productions plans for COVID-19 vaccines through 2023. A UNICEF market assessment "revealed that manufacturers are willing to collectively produce unprecedented quantities of vaccines over the coming 1-2 years."

But drugmakers have signaled the projections are "highly dependent on, among other things, whether clinical trials are successful, advance purchase agreements are put in place, funding is confirmed, and regulatory and registration pathways are streamlined."

Safe and effective vaccines are seen as essential to ending a pandemic that has so far claimed more than 864,000 lives globally.

UNICEF's new role with COVAX stems from its status as the largest single vaccine buyer in the world. The U.N. agency said it procures more than 2 billion doses of vaccines annually for routine immunization and outbreak response on behalf of nearly 100 countries.