Westfield Mayor Shelley Brindle said 11 cases announced Saturday were in the age group of 18 to 25.
Three of those cases were traced to attendance at parties while the others remain under investigation.
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Brindle said parents and families should reconsider hosting or attending graduation parties.
She asked anyone who thinks they were exposed to get tested and start self-quarantining for 14 days.
"I hope this spike serves as a reminder that we all have a responsibility to remain vigilant to mitigate community spread," Brindle said in a Facebook post. "Wash your hands. Keep a safe distance. Wear a face covering. Working together, we can get this genie back in the bottle."
Officials in New York are also monitoring an uptick in cases in Rennselaer County after several individuals traveled back to New York from Georgia.
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