New data from Moderna Inc’s (MRNA.O) large COVID-19 vaccine trial show that the protection it provides fades over time, supporting the case for booster doses, the company said in a news release on Wednesday. “This is only one estimate, but we believe that as we look toward the fall and winter, the estimated impact of waning immunity would be 600,000 additional cases of COVID-19,” Moderna President Stephen Hoge said during a conference call with investors.

However, the analysis released on Wednesday revealed that people who were vaccinated roughly 13 months ago had higher rates of infection than those who were vaccinated roughly eight months ago. The study took place between July and August, when Delta was the dominant strain. It has yet to be peer reviewed.

Briefing documents from the FDA’s analysis of Pfizer’s booster application, which were released earlier on Wednesday, indicate that one of the key issues the agency will consider is whether vaccine protection is waning.

During the two-month period from July to August, researchers identified 88 COVID-19 cases among those who had received the two shots recently, compared to 162 cases among those who had been vaccinated the previous year. Only 19 cases were classified as severe, a critical criterion for assessing waning protection.

According to Moderna, there was a trend toward a lower rate of severe cases among those who had been vaccinated recently, though the finding was not statistically significant.

Meanwhile, data from a separate study presented on Wednesday with Kaiser Permanente Southern California health system shows that Moderna’s vaccine is still effective against the Delta variant.

“We believe this will reduce the number of COVID-19 cases,” Hoge said. “We also believe that a third dose of mRNA-1273 has a good chance of significantly extending immunity for the rest of the year as we try to end the pandemic.”