Countries are suggesting boosters every 90 days

Booster shot protection against symptomatic COVID-19 caused by the Omicron variant appears to fade in about 10 weeks, according to new data from Britain.

U.K. health officials shared the data just before Christmas and noted that there haven’t been enough severe cases of the Omicron variant to calculate how well boosters protect against severe disease. But they believe the extra shots provide significant protection against hospitalization and death.

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References:
https://www.webmd.com/vaccines/covid-19-vaccine/news/20211227/covid-booster-protection-wanes-new-data

https://www.medicalnewstoday.com/articles/covid-19-protection-wanes-90-days-after-second-pfizer-vaccine