If you were to approve a coronavirus vaccine, would you approve one that you only knew protected people only from the most mild form of Covid-19, or one that would prevent its serious complications? The answer is obvious. You would want to protect against the worst cases. But that's not how the companies testing three of the leading coronavirus vaccine candidates, Moderna, Pfizer and AstraZeneca ... are approaching the problem. According to the protocols for their studies ... a vaccine could meet the companies' benchmark for success if it lowered the risk of mild Covid-19, but was never shown to reduce moderate or severe forms of the disease, or the risk of hospitalization, admissions to the intensive care unit or death. To say a vaccine works should mean that most people no longer run the risk of getting seriously sick. That's not what these trials will determine. Influenza vaccines ... reduce the risk of mild disease in healthy adults. But there is no solid evidence they reduce the number of deaths. In fact, significant increases in vaccination rates over the past decades have not been associated with reductions in deaths. Moderna and Pfizer acknowledge their vaccines appear to induce side effects that are similar to the symptoms of mild Covid-19. In Pfizer's early phase trial, more than half of the vaccinated participants experienced headache, muscle pain and chills. If the vaccines ultimately provide no benefit beyond a reduced risk of mild Covid-19, they could end up causing more discomfort than they prevent.