One Dose of J.&J. Vaccine Is Ineffective Against Delta, Study Suggests July 20, 2021 Auto Bot AstraZeneca PLC, Blood Clots, Coronavirus (2019-nCoV), Disease Rates, Food and Drug Administration, Immune System, Johnson & Johnson, Moderna Inc, Pfizer Inc, Research, Vaccination and Immunization, your-feed-science 0 Many who received the shot may need to consider boosters, the authors said. But federal health officials do not recommend second doses.