The gene taken from Harry is reproduced multiple times to be made into a vaccine. The vaccine works wonders on curing people from all muggle viruses.
The bioscience drs have run Harry's dna and discovered he has Potter and Evans matching pairs in his DNA.
Petunia Dursley ( nee Evans ) receives an official looking document in the mail. It sat their in the mailbox because she had forgotten to have their family's mailing address forward off to Australia.
Not that it mattered, only those with enough genes had the magic, but the drs discovered the potency of the gene could be increased by adding more of the gene to the stockpile.
So the bioscienctists boosted the gene therapy into a powerful vaccine that worked wonders in the muggle world. It worked quite well, but unfortunately some countries that were cash poor could not buy enough for all of their citizens.
So the more wealthier countries had more people who were quickly healed and started showing signs of magical abilities.
The Ministry's of Magic all over the world started investigating what was happening. It all came back to Harry Potter, who was now using an alias and hiding out at Potter Manor.
He had Dobby and Winky working for him. There was a lot of work needed there.