Serial infection with SARS-CoV-2 Omicron BA.1 and BA.2 following three-dose COVID-19 vaccination

Re-infections with Omicron following three doses of an mRNA vaccine has not been previously identified. This study indicates that consecutive infections with Omicron – that is, two distinct infections with the Omicron variant at least 90 days apart – are possible, even in fully vaccinated individuals with an average immune response. In January 2022, slightly more than two months after the third vaccine dose; a healthcare worker who received two Pfizer-BioNTech’s Comirnaty mRNA vaccines and boosted with a third dose of the same vaccine, was diagnosed with an Omicron sub-lineage BA.1 infection. Shortly after three months of the initial Omicron infection, this individual contracted a second infection, but, this time caused by the sub-lineage BA.2 variant. Both infections were characterised by moderate symptoms. The individual’s vaccine-induced antibody response quantified prior to the first Omicron infection was comparable to that of a group of 124 individuals similarly vaccinated during the same period. In fact; it was noted that, the individual’s concentration of receptor-binding domain (RBD) antibodies and their ability to neutralise both the original and Omicron BA.1 strains had increased compared to after the second dose of the vaccine. The first infection with Omicron BA.1 variant boosted the individual’s vaccine-induced immune response against both sub-lineages (BA.1 and BA.2), but, there was no sufficient immunity in both cases to prevent a reinfection. A second infection with Omicron BA.2 did not significantly improve previously formed immunity. This study shown that Omicron reinfections in triply vaccinated individuals are possible and it also indicates that the additional immunity conferred by Omicron may not necessarily protect against Omicron re-infection. However, vaccination was proven to effectively prevent development of a severe disease in both cases. Due to a gradual decline of immunity after vaccination, this study also highlighted the importance of booster vaccination in order to maintain sufficient level of protection against a serious illness. Additionally, using other prevention methods, such as wearing masks can prevent new infections.