This study investigated the immune responses in healthcare workers (HCWs, n=328) to different COVID-19 vaccine regimens, including mRNA vaccines (BNT162b2, mRNA-1273) and adenovirus vector vaccines (ChAdOx1, Janssen). Two doses of BNT162b2, mRNA-1273, or a combination of ChAdOx1 and an mRNA vaccine (12-week interval) induced high levels of neutralizing antibodies against the Delta and ancestral variants. A shorter 3-week interval with BNT162b2 resulted in lower neutralizing antibody titers. A third booster dose of BNT162b2 or mRNA-1273 significantly increased antibody levels and induced neutralizing antibodies against the Omicron BA.1 variant, suggesting a potential for cross-protective immunity.

Milja Belik, Pinja Jalkanen, Rickard Lundberg, Arttu Reinholm, Larissa Laine, Elina Väisänen, Marika Skön, Paula A Tähtinen, Lauri Ivaska, Sari H Pakkanen, Hanni K Häkkinen, Eeva Ortamo, Arja Pasternack, Mikael A Ritvos, Rauno A Naves, Simo Miettinen, Tarja Sironen, Olli Vapalahti, Olli Ritvos, Pamela Österlund, Anu Kantelé, Johanna Lemppainen, Laura Kakkola, Pekka Kolehmainen, Ilkka Julkunen