If I seek professsional help for mental conditions arising from the pandemic ie anxiety, I have to declare when I buy health insurance next time?

Above all, you should declare your current health to the insurer. Thereupon, the underwriter will assess your case based on your health status then.

Regardless of whether it is due to the pandemic or not, leave it to the underwriter to give you a fair evaluation. Altogether, you won't want to be caught in a situation where you get penalised for not declaring your medical condition back then.

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