Once administered, the vaccine starts to work immediately. It triggers a response from your immune system to build a memory of a specific virus or pathogen. That way your body has a faster, stronger response if it sees this pathogen again. This immune response is also the reason why you may feel flu-like symptoms after receiving a vaccine. It takes your body about 2 weeks to build a strong immune response to this specific pathogen. Just like childhood vaccines, some of the COVID-19 vaccines require boosters to achieve the best response.For the Pfizer vaccine, you must receive two doses. Currently, the second dose is recommended to be administered 21 days after the first.For the Moderna vaccine, you must receive two doses. Currently, the second dose is recommended to be administered one month after the first.For the AstraZeneca vaccine, you must receive two doses. Currently, the second dose is recommended to be administered 4 to 12 weeks after the first.For the Janssen (Johnson & Johnson) vaccine, you only need one dose.For the Pfizer, Moderna, and AstraZeneca vaccines to work best, you need to get both doses. Some immunity to COVID-19 occurs following the first dose, but the best response is obtained two to four weeks after receiving the second dose.At the moment, the recommended vaccine schedule remains the same as it is listed above. However, some people in Canada have not been receiving the vaccine according to the recommended schedule. This has mainly been due to the limited supply of vaccines, as well as the prioritization of certain populations at the provincial level.Many factors are considered when determining who should get the vaccine first, and when they should receive their second dose. There are a number of risks associated with exposure to COVID-19 that must also be taken into consideration, such as: Risk of severe death or illness Ability to physically distance Access to other measures of prevention (i.e., personal protective equipment) Vaccine supply Viral transmission rateThe National Advisory Committee on Immunization (NACI) provides information updates to the Public Health Agency of Canada. This information is then shared with decision-makers on a provincial level.Access to vaccines provincially is changing every day. We’ll be updating information on this page as it becomes available.For now, if you’d like more information, visit the Public Health Agency of Canada’s website.