The new rule was mandated by Bill de Blasio, the Mayor of New York.
Under the order, all those aged five to 11 must show proof of at least their first COVID vaccination to enter "indoor dining, fitness, entertainment and performance venues."
They must also supply proof of vaccination to partake in certain extra-curricular school events.
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After Christmas, a second provision of the policy kicks in, with all New Yorkers aged 12 and older required to show proof of two vaccine doses.
"We want to protect everyone," de Blasio said.
"We want to protect our youngest New Yorkers. We know Omicron had a lot of impact on younger folks.
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"We know we need a whole family to be safe, and if the youngest kids are safe that also helps protect our seniors."
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De Blasio leaves office at the end of the month; his successor, Eric Adams, has been noncommittal about his plans.
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Some welcomed Tuesday's ruling, seeing it as a powerful tool to protect all amid a rise in cases.
Citywide, only 78.6 percent of residents have had at least their first vaccine shot.
The number sits at 88 percent in Manhattan, but in Staten Island and the Bronx, the two boroughs with the lowest vaccination rate, the level falls to 73 percent.
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Among the youth, citywide, from 13 to 17, 82 percent have had at least one dose.
But with children aged five to 12, the rate drops to 31 percent.
Tuesday's proposition is designed to counter the low vaccination rate. Record producers Mike "Trauma" D. and Jugrnaut, known as Arkatech Beatz, tweeted: "Here is the kicker, if you want to take your 5 + year old to their favorite restaurant or event space they will have to show proof of vaccination."
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One person suggested it would significantly harm Broadway theaters saying, "Lol… can't wait for them to go under.
"Only 16% of kids have been vaccinated. Their ticket sales are gonna TANK. Vaccinated or not the large majority does NOT believe in mandates. These Liberal-controlled cities are going to prove a great point: none of this works."
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Another added: "When Dogma is more important than actual science. Congrats, Broadway!"
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One anti-vax mother tweeted: "Insanity! Finland makes it illegal to vaccinate kids under 12 b/c the chance of severe adverse reaction is significantly higher than severe covid! UK not vaccinating under 12s either! Also if your child hurts u R own yr own. U have 8% chance of winning financial compensation!"
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