Ten months ago, the results of large clinical trials appeared almost too good to be true: Two messenger RNA vaccines reduced symptomatic COVID-19 cases by more than 90% in almost every group that got them.

Now, subtle differences between the Pfizer Inc.-BioNTech SE and Moderna Inc. vaccines are emerging across patient groups over time. One small U.S. study found waning levels of antibodies with Pfizer’s vaccine, particularly in an older group of people. And a larger study from Belgium found that Moderna’s shot may generate more antibodies than Pfizer’s.

But what this all means in the real world is still unclear. While hundreds of millions of doses of vaccines have been administered around the world, researchers are still working to understand the nuances of how long their protection lasts, and how it differs from one person to another.