MSN short: "Many vaccines—including those for hepatitis A and B,
measles, chickenpox, and human papillomavirus—do prevent people both from becoming ill and from passing the pathogen to others."
They don't
know, but can draw inferences from similar diseases, to inform an assessment of likelihood. Such assessments will be taken into account (along with much else) when national and regional precaution regimes are being revised. It's entirely right for scientists to say they don't know (if they don't) but life (generally not individually) and societal choices are not monochrome.
Anyway, longer from MSN:
Susanna Naggie, an associate professor of medicine in the Duke University School of Medicine who specializes in infectious diseases: “With flu there is asymptomatic disease, but not at the level we see with SARS-CoV-2. This makes it particularly important to understand whether COVID-19 vaccines will prevent asymptomatic infections."
Jeffrey Bethony, a professor of microbiology, immunology, and tropical medicine at the George Washington University School of Medicine and Health Sciences who works on vaccines for parasitic diseases and HIV:
"Many vaccines—including those for hepatitis A and B,
measles, chickenpox, and human papillomavirus—do prevent people both from becoming ill and from passing the pathogen to others. Generally, we believe that if you have a vaccine that prevents disease, you’re likely preventing infections as well, but you can’t assume that that’s 100 percent [the case].
"Some pathogens can infect and reproduce in vaccinated people for short periods of time without making them sick, including . . . meningitis and
whooping cough. This is also a problem for vaccines under development for parasitic illnesses such as malaria, schistosomiasis, and hookworm infection. The vaccine protects people against the most serious clinical manifestations of the disease but it doesn’t entirely stop infection. You still might have a person who is mildly infected, and they’re still able to spread the disease.”