The term stands for "Those Who Are Decided Against Learning", because those I engaged with were not "vaccine hesitant" in any sense.
They were mostly dedicated to pushing an agenda of anti-science, anti-logic, anti-politeness underpinned by memes and quote-cards highlighting every rare bad outcome from a vaccine, every side (well known and mostly minor) adverse event and never once mentioning all the lives that vaccines have saved and protected yesterday, today and in the future.
By all means describe this as name calling - but go spend some time engaging with TWADALs first - some quality time. Prove to me that they are open to learning anything new. Show me that after they drop into your social media neighbourhood unannounced and spew unsolicited, unverifiable out-of-context untruths about vaccines killing people, collect a few others to pile on, call you a shill for big pharma...that they will then concede any of their points when you try to engage them in a calm discussion.
Vaccine objectors? Vaccine benefit deniers? Those Who Are Decided Against Learning? https://t.co/RioZMOhKrF— Ian M Mackay, PhD (@MackayIM) August 26, 2016
Those Who Are Decided Against Learning #TWADAL
— Ian M Mackay, PhD (@MackayIM) August 28, 2016