The 23-year-old trainee barrister went missing in Thailand on the final leg of a solo backpacking trip around the world.
She was due to fly home 10 days ago to spend Christmas with her family but failed to catch her flight to Heathrow airport.
Her parents, Stuart and Jackie, both 49, have since flown out to Ko Samui, one of a group of islands 400 miles south of Bangkok, to join Interpol in the search for their daughter.
Johanne's brother, sister and other relatives were anxiously awaiting news at the family home in Wincle, Macclesfield, Cheshire.
Inspector Bob Bates, of Cheshire police, last night urged holidaymakers returning home from the Far East to jog their memories.
"We are certain that other backpackers and holidaymakers returning home from Thailand will have seen this girl on their travels and could hold vital information about her whereabouts," he said.
"It is so out of character for her to go off without word and we are extremely concerned for her well-being."
Johanne, a law student at York University, left home in September and travelled alone to Canada, Australia, New Zealand and Fiji before arriving in Thailand last month.
The family's only clue is a postcard she sent a friend on 8 December outlining her plans to explore the cluster of islands south of the mainland.
Johanne's aunt, Wendy Millard, said: "We are all sick with worry.This is the first time Johanne has gone off on her own. She has kept in touch with us all the time she's been away - either by telephone or postcard.
"But nobody knows what has happened to her now. It's so out of character for her to just disappear without telling anyone.
"She was determined to go backpacking around the world on her own and saw it as quite a challenge.
"Her parents flew out ... on Thursday to look for her because they just couldn't sit at home waiting for news.
"Johanne met up with several friends on her travels and we can only hope that one of them can give us a lead.
"Johanne was really looking forward to coming home for Christmas. She'd travelled alone to various countries on her world trip but always kept in touch with us on the way.
"She's such a happy, friendly, gregarious person and very, very responsible. It is not like her to just disappear like this."Reuse content