The WHO continuously reviews available data on SARS-CoV-2 variants of concern. For this version, the global epidemiological situation of the COVID-19 pandemic as of 21 January 2022 – at a time when the Omicron VOC had been identified in 171 countries across all six WHO Regions and was rapidly replacing Delta worldwide – was considered Omicron has a substantial growth advantage, higher secondary attack rates and a higher observed reproduction number than Delta. There is now significant evidence that immune evasion contributes to the rapid spread of Omicron. Other factors may be a shorter serial interval (by about 0.8 to 1.2 days compared to Delta) and potential increased intrinsic transmission fitness . There is growing evidence that with Omicron, there is lower vaccine effectiveness (VE) against infection and symptomatic disease soon after vaccination compared to Delta. There is also evidence of accelerated waning of VE over time of the primary series against infection and symptomatic disease for the studied vaccines. Further studies are required to better understand the drivers of transmission and declining incidence in various settings. These factors include the intrinsic transmission fitness properties of the virus, degree of immune evasion, vaccination coverage and level of vaccine-derived and post-infection immunity, levels of social mixing and degree of application of public health and social measures (PHSM).
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