COVID-19 vaccine effectiveness against omicron infection fell dramatically after six months for people who only got their primary series, according to a new analysis published Wednesday. The study may not accurately reflect vaccine effectiveness in the United States because researchers included many of the most common vaccines used around the world, including Pfizer-BioNTech, Moderna, AstraZeneca and Sinovac. The researchers, however, did not look at the updated bivalent vaccines that target the original strain of the virus as well as BA.4 and BA.5, which are subvariants of omicron.