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Omicron
Omicron is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. This letter is derived from the Phoenician letter ayin. In classical Greek, omicron represented the sound [o] in contrast to omega [ɔː] and ου [oː]. In modern Greek, omicron represents the mid back rounded vowel /o̞/, the same sound as omega. Letters that arose from omicron include Roman O and Cyrillic O.
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A meta-analysis of 65 studies finds that prior COVID-19 infection confers substantial protection against pre-omicron variants, but not Omicron BA.1. Regardless, the authors conclude the level of protection from past infections is equal to, if not greater than, that provided by two-dose mRNA vaccines