"For individuals planning a trip to another country, such as Vietnam, which organization is the best source for information on which vaccines might be necessary?"

Passporthealthusa, world health organization and CDC are the best website.

Explanation:

These websites provide a good insight to the travelers what precautions they must take to reduce the threat of catching a decease in the foreign country. These are the best source of information to know what sort of deadly deceases are common in Vietnam. Corona-virus is there so be more cautious.

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