Ever since the US vaccine rollout officially commenced in December, a question that has been top of mind is whether the messenger RNA (mRNA) vaccines created by Moderna and Pfizer/BioNTech would perform as well under real-world conditions as they did in the laboratory. Though the clinical trials of mRNA vaccines proved they were largely effective at preventing disease, their ability to prevent infection remained unknown.

Thanks to a study published yesterday by the US Centers for Disease Control and Prevention, we now have our answer. According to the study, which was conducted on nearly 4,000 healthcare workers, first responders, and other essential workers at the frontlines in eight locations across the country, the mRNA vaccines are 90 percent effective at preventing infection. That means in addition to stopping the development of Covid-19 symptoms, they can stop the disease from spreading from one person to another, too.

In December 2020, the study’s participants, who lived everywhere from Phoenix, Arizona to Portland, Oregon, began receiving doses of the Moderna or Pfizer-BioNTech vaccines. Almost three quarters had at least one dose by March 2021. To cast a wide net that could catch both symptomatic and asymptomatic infections, researchers required their subjects to test themselves for Covid-19 week in and week out using standard nasal swabs.

In the group of unvaccinated workers, 161 ended up developing infections. By contrast, in the group of vaccinated workers, only 16 who received one dose caught the virus before they could get their second, while just three received both but caught it in the two-week period following the second dose. It was those three infections that brought the efficacy of the full two-dose regimen of the mRNA vaccines down to 90 percent. The mRNA vaccines aren’t foolproof, in other words, but given the circumstances they’re performing remarkably well.

The results of this study, of course, are only preliminary, with investigations of greater depth and breadth to come. And though we hope the Covid-19 vaccines that use other platforms, like the adenovirus vaccines created by Johnson & Johnson and AstraZeneca, prove to be just as successful, they fall outside the bounds of this particular report.

That said, two issues worth noting remain for all vaccines that the mRNA vaccine study didn’t address but forthcoming research might. The first is how long protection from disease and infection lasts. One recently published preprint study used a predictive model to answer this question in theory, calculating the duration of vaccine-mediated antibody responses—their indicator of immune protection of choice—based on already available data. Their best guess is that duration of protection correlates with a vaccine’s initial efficacy, but without real-world data we can’t be sure.

The second issue is whether the protection will hold up against new variants of SARS-CoV-2. While the report never explicitly states that the participants contracted the original strain, if the question of viral variation wasn’t broached we can only assume it wasn’t relevant at the time. Various laboratory studies have found that the antibody-rich plasma of people who receive the mRNA vaccines loses its potency when pitted against new variants of the virus, particularly B.1.351 and P.1, which contain mutations now associated with a greater potential for immune escape.

Overall, the news that the mRNA vaccines prevent infection in addition to disease is excellent. What it isn’t is a reason to let down our guard. As vaccine developers collect more data on how their products operate in real-world contexts, they must continue to release it in a timely fashion. They must also address the key questions that people who are pinning so many hopes on their vaccines continue to have. Important though it may be to celebrate every milestone, so long as this disease continues to spread among us and take lives, we must tackle every unresolved mystery with great haste and rigor. Nobody wants to be left in the dark—least of all when the light at the end of the tunnel seems increasingly near.