OTTAWA -- Canada's chief public health officer says the fight against vaccine-related misinformation on social media is being further complicated by the emergence of online bots designed to sow mistrust.

Dr. Theresa Tam says very divisive bots have been spreading false claims about vaccinations -- and she adds misinformation on social media is considered at least partly to blame for the 300-per-cent global increase this year in measles.

Tam says bad actors online can be very effective in sowing mistrust among parents, planting doubts and persuading people that a vaccination debate is underway among credible health experts, when in fact, she notes, there is no debate.

Speaking to reporters after an event Friday in Ottawa, Tam says there are concerns Canada's vaccine coverage could slide from its current level -- which she described as not high enough, though not terrible.

She says anyone is potentially susceptible to misinformation online because even a short exposure to it can change someone's mind, and she adds it's of particular concern considering 20 to 30 per cent of parents have questions about vaccines.

Tam says the key is to make sure parents with questions get science-based, credible answers from health providers.