Chaffer had been head of media and press secretary to the Minister of the Cabinet Office since October. Prior to that she worked for a number of NHS trusts in comms and PR roles since 2008, and before that for the trade body Oil & Gas UK.
She took up her new role on Wednesday.
HealthUnlocked aims to support people with health conditions or questions to find others with similar health backgrounds, allowing them to take on day-to-day health concerns together – it runs more than 600 communities and has had more than three million monthly users. It was founded in 2009.
Chaffer said: "We have an important role to play within health and supporting better outcomes. I look forward to taking the digital healthcare agenda forward and making a real difference to people's lives."
Jorge Armanet, CEO of HealthUnlocked, said: "We’ve been working relentlessly over the past few years to create a unique platform that is globally delivering better patient outcomes and dramatic savings to providers. Alexa is joining us at a crucial stage when the company is rapidly growing. I’m looking forward to seeing how her great experience and skills will impact our journey and those of all the millions of patients who use our platform every month."