PMC:7126544 / 65964-66114 JSONTXT

{"project":"LitCovid-PD-CLO","denotations":[{"id":"T547","span":{"begin":123,"end":124},"obj":"http://purl.obolibrary.org/obo/CLO_0001020"}],"text":"s. However, the difficulties and challenges of developing an effective vaccine have been discussed earlier, and the key to a successful vaccine progra"}