In a new article for the Lancet medical journal, two University of Hong Kong researchers caution that even at the conclusion of Phase III clinical trials, important questions will remain about how effective vaccines are in stopping viral transmission of COVID-19.

Gabriel Leung, an infectious disease epidemiologist, and Malik Peiris, a public health virologist, argue that even though multiple vaccine makers have reached Phase III clinical trials at a record pace, scientists are currently studying only part of the equation in gauging how a vaccine may help bring an end to the global pandemic.

Currently, they say, vaccines are being evaluated only for how they prevent those receiving the vaccine from developing COVID-19 and not for how a vaccine might be able to reduce viral transmission.

“The impact of these COVID-19 vaccines on infection and thus transmission is not being assessed,” they write. “We cannot assume COVID-19 vaccines, even if shown to be effective in reducing severity of disease, will reduce virus transmission to a comparable degree.”

In other words, vaccine makers may determine whether a vaccine can protect those injected from getting sick, but not necessarily whether the vaccine stops those inoculated from spreading the disease to others. So even if a vaccine reaches market—which could happen as early as the end of this year—there may be little evidence as to how effective the vaccine is at stopping the spread of COVID-19.

Given this hole in the research, policymakers and the general public should calibrate their expectations in terms of how a COVID-19 vaccine rollout will return the world to a pre-pandemic “normal,” Leung and Peiris say.

U.S. public health experts also have recently tried to dampen public expectations about the effectiveness of a potential vaccine.

“I might even go so far as to say that this face mask is more guaranteed to protect me against COVID-19 than when I take a COVID-19 vaccine, because [the vaccine] may be 70% [effective],” Robert Redfield, director of the Centers for Disease Control and Prevention, said in a congressional hearing on Sept. 17.

U.S. President Donald Trump called Redfield’s statement “incorrect,” but other infectious disease experts backed up Redfield’s claim. The Food and Drug Administration requires only that a COVID-19 vaccine is proved to be 50% effective for those injected to be approved.

Despite potential limitations, Leung and Peiris argue that the general public should not be hesitant to get vaccines when they come to market.

“COVID-19 vaccines are needed” to protect vulnerable populations, Leung and Peiris write. But vaccines are “only one tool in the overall public health response to COVID-19 and unlikely to be the ultimate solution that many expect.”