It's value depends on what you mean by "50% effective." Does that mean 50% reduction in severity because they get partial immunity or does it mean 50% of people still get a full blown case of the disease? Either way, that is AN ENORMOUS benefit. Any boost in immunity or treatment can easily make a huge difference between the numbers treated without hospitalization, the number treated without intubation, the number surviving. And a major reduction in the time to recovery for those who still get it. People need to keep their expectations bounded but also recognize that a partial benefit would be big benefit. Similarly we have flu vaccines and pneumonia vaccines but those only reduce a person's chances of getting a full blown case. For most people in reasonably good health a boost to their immunity gives good protection. The flu vaccine certainly has not in any way even come close to stopping the spread of influenza and their are plenty of people walking around every year with the flu who had a flu shot. But the vaccine may still have kept them out of the flat-lining zone. My point being: Don't knock partial immunity if some vaccine can provide it.