rafay been of the berman institute of bioethics at johns hopkins university and a member of the world health organization's covert 19 vaccines working group thank you for your thoughts. my pleasure and now it is over to our resident coronavirus experts to w. science correspondent derrick williams answering your questions about the pandemic . if a vaccine is not 100 percent effective can you still catch the virus off to vaccinate ssion. first of all no vaccine that we've developed for any diseases ever provided perfect protection to everyone although although a few like those for measles or for polio come very close but because every person's immune system is different in some people exposed to sars cove to the new vaccines will not stop the corona virus in its tracks trials in healthy volunteers indicate however that the candidates now being authorized for use in countries around the world are highly effective at doing so in most people it's kind of complicated but a simplified way of quantifying the efficacy demonstrated in trials would be to say that if you haven't been vaccinated y