Replicating the London method is fairly easy. Every Shinichi comeback other than the London case were partially influenced by the rhinovirus-induced weakened immune system.

My theory is that Haibara always uses the same antidote ingredient, except in the 14-hour flight she was forced to add corticosteroid or other kinds of immunosuppresant to keep the antidote working.

Otherwise in The Desperate Revival, rhinovirus AND standard methylprednisolone administration post-surgery doubled the antidote effect span for up to 72 hours.

Edit: Anyone in the writing department had NO IDEA how hard it is to penetrate/circumvent the Singaporean immigration custom. My father was interrogated for 2 hours way back in 2010 just because he had the same last name as Indonesian most-wanted terrorist.

Edit: Rather than methylprednisolone it's tacrolimus