Johns Hopkins Medicine is moving forward with clinical trials for a potential coronavirus therapeutic at sites across the country, including two in the District. The Baltimore-based health system is studying convalescent blood plasma as both a treatment and defense against Covid-19, after filing its investigational new drug application with the Food and Drug Administration in the spring. But “we quickly saw that we can’t do it by ourselves‚” said Dr. David Sullivan, professor of molecular microbiology and immunology at the Johns Hopkins School of Medicine. “We need to utilize sites all over the country.”