After one dose of a highly effective messenger RNA (mRNA) vaccine, just 17% of those patients churned out protective antibodies against the pandemic coronavirus, and after the standard two doses, only 54% did. A third dose of vaccine may help: Among 24 organ transplant patients who had no antibodies after two doses, eight people generated protective antibodies after they sought out a third on their own. Six people who had few antibodies against the coronavirus after two doses all wound up with high levels after a third shot, the researchers reported today in the Annals of Internal Medicine. In the United States, people with organ transplants number about 500,000, but they aren’t the only ones worrying about how well the vaccines are working for them—others include people with autoimmune diseases and those with cancer who got COVID-19 vaccines while their immune system was suppressed by chemotherapy. “We have a strong biologic rationale for a third dose in specific populations,” says Ravi Parikh, a health policy researcher and medical oncologist at the University of Pennsylvania. His patients haven’t asked him yet about third doses, but he imagines himself supporting that strategy for some. When it comes to people with cancer, Parikh is not deeply worried about the effectiveness of two doses. Last month, a study in JAMA Oncology reported that 90% of a group of cancer patients on chemotherapy and other drugs produced antibodies after two doses of Pfizer’s mRNA vaccine. Read more at Science Magazine.