The effectiveness of the three COVID-19 vaccines available in the United States has declined in recent months, with protection against infection falling under 50 percent for two of them after six months, according to a new study. Moderna’s COVID-19 vaccine dropped from 89.2 percent in March to 58 percent in September, researchers found. During the same period of time, Pfizer’s COVID-19 vaccine dropped from 86.9 percent effective to 43.3 percent, and Johnson & Johnson’s shot dropped from 86.4 percent to 13.1 percent. Dr. Stephen Hahn, the head of the Food and Drug Administration during the final portion of the Trump administration, said last year that the agency wouldn’t authorize COVID-19 vaccines that weren’t at least 50 percent effective against infection. Researchers also found that the vaccines’ protection against death waned after six months, particularly among older people. Instead of comparing the effectiveness in March and September, though, they used the data …