Jolanta Gawlik, left, gives Juliana Cepeda the Pfizer COVID-19 vaccine in New York.Mark Lennihan/AP Photo Booster shots appear to be at least 90% effective against hospitalization for Delta and Omicron. But Omicron poses a greater challenge to vaccine protection, according to two new CDC studies. Omicron increased the odds that vaccinated or boosted people would get infected relative to Delta. COVID-19 booster shots were effective at preventing hospitalization and death before Omicron started spreading in the US.