Pfizer, the pharmaceutical company that created one of the first Covid-19 vaccines to be approved, has been making a hard sell for emergency approval of boosters – additional doses given to those already vaccinated, especially immunocompromised adults.

But in private meetings with Pfizer on Monday, senior US officials said they needed more data – prompting the latest debate over how to curb a pandemic which has claimed more than 620,000 lives in the country. Last week, the US health department also rebuked Pfizer for pressing for a booster shot, and Anthony Fauci, Joe Biden’s chief medical adviser, has said there isn’t enough evidence to support needing a third shot.

“It was an interesting meeting. They shared their data. There wasn’t anything resembling a decision,” Fauci said in a Monday evening interview with the New York Times. “This is just one piece of a much bigger puzzle, and it’s one part of the data, so there isn’t a question of a convincing case one way or the other.”

In the US, almost half of the population is fully vaccinated, while a little over half has received one dose, according to data from the Mayo Clinic. Still, vaccination rates lag in huge swaths of the country, giving the virus more opportunities for community outbreaks.

Pfizer’s experts have pointed to Israel, where the government has decided to give a third Pfizer vaccine shot to vulnerable adults. But leaders from the World Health Organization and other organizations have pushed back, highlighting the vast lack of access and inequality in global vaccine distribution. More than 3.4 billion people have been vaccinated worldwide, but some countries, such as India, have rates as low as 5%.

The debate over booster shots is the latest in the many public health decisions the Biden administration has faced since January. With the country largely relaxing Covid-19 rules and opening the economy, the path forward continues to be difficult, with emerging science being incorporated in real time.

In next steps, Pfizer says it will submit more evidence to the government. The Centers for Disease Control and Prevention, meanwhile, will further study breakthrough infections – which happen when people who are vaccinated contract Covid-19.