Would they work better in combination? And would the companies allow that to happen?

Theoretically, yes. And it’s unlikely.

Combinations of antiviral drugs are the standard treatment for people with HIV because it reduces the risk of resistance caused by mutations in the virus. Since the Pfizer and Merck pills attack SARS-CoV-2 differently, using them in combination might offer the same protective benefit for patients with Covid, said Céline Gounder, a physician and infectious disease expert at New York University’s Grossman School of Medicine.

“The challenge is that since these drugs are developed by different companies, neither Merck nor Pfizer is incentivized to run a combination therapy trial,” said Gounder. “However, the National Institutes of Health or others could do that, and I think it’s really important that they start to develop a combination therapy.”

Nahid Bhadelia, the founding director of the Center for Emerging Infectious Diseases Policy and Research at Boston University, also supports the idea of clinical trials to test combination treatments. Combination therapy is standard in HIV because patients are treated chronically — the virus is never fully cleared. SARS-CoV-2 is cleared in most people, but it’s also a faster-evolving virus.