bioRxiv 2022.05.08.491108 (2022). 4.0) license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. This license does not apply to figures/photos/artwork or other content included in the article that is credited to a third party; obtain authorization from the rights holder before using this Dansylamide material. Abstract Despite the remarkable efficacy of COVID-19 vaccines, waning immunity and the emergence of SARS-CoV-2 variants Dansylamide such as Omicron represents a global health challenge. Here, we present data from a study in nonhuman primates demonstrating durable protection against the Omicron BA.1 variant induced by a subunit SARS-CoV-2 vaccine comprising the receptor binding domain of the ancestral strain (RBD-Wu) on the I53C50 nanoparticle adjuvanted with AS03, which was recently authorized for use in individuals 18 years or older. Vaccination induced neutralizing antibody (nAb) titers that were maintained at high concentrations for at least 1 year after two doses, with a pseudovirus nAb geometric mean titer (GMT) of 1978 and a live virus nAb GMT of 1331 against the ancestral strain but not against the Omicron BA.1 variant. However, a booster dose at 6 to 12 months with RBD-Wu or RBD- (RBD from the Beta variant) displayed on I53C50 elicited high neutralizing titers against the ancestral and Omicron variants. In addition, we observed persistent neutralization titers against a panel of sarbecoviruses, including SARS-CoV. Furthermore, there were substantial and persistent memory T and B cell responses reactive to Beta and Omicron variants. Vaccination resulted in protection against Omicron infection in the lung and suppression of viral burden in the nares at 6 weeks after the final booster immunization. Even at 6 months after vaccination, we observed protection in the lung and rapid control of virus in the nares. These results highlight the durable and cross-protective immunity elicited by the AS03-adjuvanted RBD-I53C50 nanoparticle vaccine. INTRODUCTION Waning immunity coupled with the continuing emergence of immune-evasive variants represents a major challenge in controlling the coronavirus disease 2019 (COVID-19) pandemic. The efficacy of the highly effective mRNA vaccines has been shown to decrease 20 to 30% by 6 months after a two-dose vaccine series (1, 2). The efficacy declined more precipitously against Omicron, a variant highly resistant to vaccine-induced antibodies (3C5), reaching 8.8% after the two-dose Pfizer-BioNTech mRNA vaccination. The Dansylamide waning efficacy thus mandates a booster vaccination despite the fact Dansylamide that about 40% of the worlds population are yet to receive full vaccination. We recently reported the results of a study in nonhuman primates (NHPs) in which we compared the immunogenicity and protective efficacy of the receptor binding domain (RBD)CI53C50 nanoparticle immunogen formulated with five different adjuvants (6). Administering the RBD-I53C50 vaccination with AS03, an oil-in-water emulsion containing -tocopherol, elicited the most potent and broad neutralizing antibody (nAb) response, as well as substantial T cell responses, and conferred protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) challenge in the upper and lower airways. Furthermore, vaccine-induced nAbs persisted for at least 6 months after vaccination. In addition, AS03-adjuvanted RBD-I53C50 vaccination elicited potent nAb response in humans in a phase 1/2 clinical trial (7). Recently, the AS03-adjuvanted RBD-I53C50 met its primary end point in the phase 3 clinical trial and was FLN1 approved by Dansylamide the Korean Ministry of Food and Drug Safety for use in individuals 18 years or older. In the present study, we evaluated the durability of immune protection after a booster immunization with RBD-Wu or RBD- against the immune-evasive Omicron BA.1 variant. RESULTS AS03-adjuvanted RBD-I53C50 vaccination elicits robust and durable antibody responses The study involved four groups of male rhesus macaques. The first group of five animals were immunized thrice with RBD-Wu + AS03, at days 0 and 21, followed by a final booster about 6 months later (group RBD-Wu/RBD-Wu/RBD-Wu; Fig. 1A). The second and third groups were from our previous study (6) in which one group of five animals received two doses.