NACI says mixing AstraZeneca, mRNA vaccine doses safe and effective

No matter your first dose, it's now recommended that any mRNA vaccine as a second dose is safe for adults.

By Michael Ranger, Lucas Casaletto

As second doses begin to ramp up nationwide, new guidance on COVID-19 vaccines has been released.

The National Advisory Committee on Immunization (NACI) updated its guidance to recommend that the second shot of an mRNA vaccine, Moderna or Pfizer, can be the follow-up to the first dose of AstraZeneca.

NACI says individuals that received the first dose of Pfizer or Moderna should be offered the same mRNA shot. If supply does not allow for that, the health committee says those who received the first shot of Pfizer could, then, get a second dose of Moderna or vice versa.

NACI recommends that:

  • Persons who received a first dose of the AstraZeneca/COVISHIELD vaccine may receive either AstraZeneca/COVISHIELD vaccine or an mRNA vaccine (Pfizer-BioNTech or Moderna) for their second dose unless contraindicated.


  • Persons who received a first dose of an mRNA vaccine (Pfizer-BioNTech or Moderna) should be offered the same mRNA vaccine for their second dose. If the same mRNA vaccine is not readily available or unknown, another mRNA vaccine can be considered interchangeable and should be offered to complete the vaccine series.


NACI says they considered multiple factors before Tuesday’s official announcement, such as the risk of severe blood clots with low blood platelets associated with the AstraZeneca viral vector vaccine but not the mRNA vaccines, and the possibility of increased short-term side effects when using mixed COVID-19 vaccine schedules.

“mRNA and AstraZeneca/COVISHIELD vaccines are both available in Canada and there will be sufficient supply of both types of vaccine to provide second doses,” Health Canada said.

“Getting the same vaccine for the first and second dose or a mixed schedule are both considered valid
options, and both will count as a completed series.”

Health Canada suggests people should consider talking to a healthcare professional for help with understanding information to support “informed individual decision-making on vaccination.”

Federal officials held a news conference at noon on Tuesday to provide an update on COVID-19, including NACI’s guidance on the interchangeability of vaccines.

NACI had been holding out on more research from around the world. Now, countries like the U.K. and Spain have found mixing and matching is indeed safe and effective.

Data and evidence from those studies determined that a first dose of the AstraZeneca vaccine followed by a second dose of mRNA vaccine (Pfizer and Moderna) has a “good safety profile at shorter (4-week) and longer (8- to 12-week) intervals.”

“There is evidence that providing an mRNA vaccine after AstraZeneca vaccine will boost the immune response, which is what we expect from a second dose,” Health Canada wrote.

Health Canada says there is a possibility of increased short-term side effects when using mixed COVID-19 vaccine schedules. These include headache, fatigue and feeling generally ill.

“This was particularly noted with a short interval of 4 weeks between the first and second dose,” they wrote. “These side effects are temporary and resolve without complications.”

Prior NACI guidance stated that mixing should only be an option between mRNA vaccines if the same first dose is unavailable.

Last week, the panel changed its guidance on when to give people their second dose. NACI says now that there’s a greater vaccine supply flowing into Canada, second shots should be given out as soon as possible.

All provinces paused the use of AstraZeneca for first doses in May as reports of vaccine-induced blood clots rose, but are anxiously waiting for the NACI advice on what to do with second doses.

Health Canada says the rate of VITT after the second dose of the AstraZeneca vaccine appears to be lower than with the first dose but “has increased over time,” with current estimates of approximately 1 per 600,000 people vaccinated.

Manitoba is the first province to allow vaccine mixing with the province announcing Monday that any resident who received the first shot of AstraZeneca can get a second dose of Pfizer or Moderna.


Ontario announced last week that it would shorten the minimum interval for the Pfizer and Moderna vaccines to just four weeks, starting with those 80 and older.

The accelerated second dose plan aims to have the majority of Ontario residents who choose to get the vaccine be fully vaccinated by the end of summer.

Health Canada has added another month to the shelf life of thousands of doses of the AstraZeneca vaccine that were due to expire in the province on Monday.

The second dose of AstraZeneca is being offered to people who received their first shot between March 10 and March 19. The shots are being offered through select pharmacies at a 10-week interval in order to make use of them before expiration.

More than 21 million Canadians have at least their first dose of a COVID-19 vaccine but just over two million of those have been fully vaccinated.

Canada is set to receive 2.9 million COVID-19 vaccine doses this week thanks to an increase in planned deliveries from Pfizer.

Pfizer is increasing its deliveries to 2.4 million doses per week in June. The other 500,000 shots due to arrive this week will come from Moderna.

Top Stories

Top Stories

Most Watched Today