LONDON – There was a substantial fall in neutralization titers against the Omicron variant of SARS-CoV-2 in the stored sera of people who had received two doses of either Astrazeneca plc’s or Pfizer Inc./Biontech SE’s COVID-19 vaccines, with some samples failing to neutralize the virus at all, according to the latest data from the U.K. Com-Cov study.