For Canadians who received the first dose of Moderna or Pfizer, NACI recommends that they can now use either of the two injections as the second dose—because they both use similar mRNA technology—if the same first dose is not available or unknown .

The updated NACI guidelines are based on emerging research Spain with United Kingdom It was found that mixing and matching AstraZeneca and Pfizer vaccines are both safe and effective in preventing COVID-19.

CBC News first reported on the details of the proposed changes on Tuesday morning based on information from the sources who had direct knowledge of the decision. These sources did not want to be named. Officials of the Public Health Agency of Canada publicly confirmed the details of the change in guidelines at a press conference on Tuesday afternoon.

Canada’s Chief Public Health Officer, Dr. Theresa Tam, said that the decision to alternate combinations of mRNA vaccines is “not new.” In the past, the same principles have been applied to different types of vaccines—including influenza and hepatitis A vaccines.

“This is not a new concept, so the multi-dose series vaccines provided by the manufacturer are what the public health department has used for many other vaccines for a long time,” Tam said. “When vaccine plans and supplies change, this is not unusual.”

The updated NACI recommendation states that “currently there is no data on the interchangeability of COVID-19 mRNA vaccines”, but Tam said that “ongoing research” on the effectiveness of the method has not yet been published. She said the decision was made based on the similar composition of the vaccine, both of which target the spike protein of the coronavirus.

These recommendations will have a significant impact on the promotion of vaccines in Canada. The previous NACI guidelines stated that a series of vaccinations starting from AstraZeneca should be followed up with the same type of vaccine and should be used interchangeably only when the same first dose is not available. mRNA vaccine or unknown.

Due to AstraZeneca’s supply problems and a rare but serious blood clot that may be caused by the injection of the vaccine, some provinces have taken actions to combine different injections, which are called vaccine-induced immune thrombosis Thrombocytopenia (VITT), NACI also cited this point. As the reason for its decision in its guidelines.

According to statistics, as of May 22, more than 13 million people in Canada have received at least one dose of Pfizer vaccine, and as of May 22, more than 3.5 million people have been vaccinated with Moderna vaccine and more than 2.1 million people have been vaccinated with Aspen Likang vaccine. The latest federal government data available.

Watch | Infectious disease experts say provinces are expected to recommend mRNA vaccines after AstraZeneca

Tam stated that NACI believes that the “large amount” of data concerning the use of mRNA vaccines to focus on AstraZeneca is sufficient to update its guidelines, and Canadians need to look at “all the information before them” regarding the risks of AstraZeneca vaccines.

“At the rate of… [VITT] It seems to be lower after the second dose than after the first dose, but this situation may increase over time, and I think people need to know this type of information,” she said. “Now, 1 in every 600,000 people Received the second dose is to have [VITT] -But this may change. ”

Professor Alyson Kelvin, an assistant professor at Dalhousie University and a virologist at the Canadian Center for Vaccinology and Saskatoon Vaccines and Infectious Diseases, said the guidelines are “appropriate” given the available data.

Kelvin said that she believes that Pfizer and Moderna vaccines can be effectively interchanged because of the similarities between clinical trial data and two real-world studies based on mRNA vaccines.

“I don’t have any concerns about mixing and matching, knowing the ingredients of the vaccine,” she said, adding that Pfizer’s reported mild symptoms are slightly higher than Pfizer’s non-serious side effects. “They are quite.”

Provinces began to change vaccine dosage orders

Dr. Zain Chagla, an infectious disease doctor at the Hamilton Hospital of St. Joseph Healthcare and an associate professor at McMaster University, said that based on existing international data, NACI’s approach to mixing and matching COVID-19 vaccines is reasonable.

“[Pfizer and Moderna] They are interchangeable, and I doubt there will be any difference between the two,” he said. “Their immune responses should be very similar.

Manitoba health officials announced on Monday that residents who received the first dose of AstraZeneca vaccine Can receive a second dose of Pfizer BioNTech or Moderna vaccine If they meet the provincial eligibility requirements.

Watch | Manitoba allows mixed vaccines:

Manitoba became the first province to allow people who received the first dose of AstraZeneca-Oxford COVID-19 vaccine to receive the second dose of Pfizer or Moderna. 2:09

BC Health Officer Dr. Bonnie Henry said at a press conference on Monday that British Columbians who have received the initial dose of Moderna or Pfizer will have the option of receiving the mRNA vaccine as a second injection. Expect more guidance this week At AstraZeneca.

Quebec officials announced in April The province will begin to vaccinate its most vulnerable residents with a second dose-even if the second dose does not match the first dose.

Public Health Agency of Canada (PHAC) It is now estimated that the VITT rate of Canadians vaccinated with AstraZeneca is between 1 in 83,000 and 1 in 55,000, and the mortality rate is between 20% and 50%, but as more data becomes available, this number may Will change.

PHAC said in a statement to CBC News on Thursday that of the 2 million doses administered in Canada to date, there have been 27 confirmed cases of VITT, and 5 of them have died.

Chagla said that Canadians who hope to get the second dose of AstraZeneca will get it in the next few weeks, but said that once Pfizer and Moderna injections become more readily available in Canada, it may be phased out.

“The second dose makes sense,” he said. “With so many vaccines entering Canada, the first dose is starting to get a little tricky.”

Kelvin said that the future use of AstraZeneca vaccines needs to “go hand in hand” with the development of disease screening mechanisms to determine who is most likely to develop rare blood clots.

“Before we can do this, we need to at least provide information about your overall risk and discuss the use of other vaccines as your second dose,” she said.

“After the AstraZeneca vaccination, it is important to move forward to determine the profile of who may be at risk of severe blood clots.”

Early research shows that mixed vaccines are safe and effective

The Spanish study that provided guidance for NACI analyzed 670 volunteers between the ages of 18 and 59 who had received the first dose of AstraZeneca vaccine, and 450 of them had received Pfizer vaccine.

The study found that those who received the first dose of AstraZeneca vaccine and the second dose of Pfizer vaccine Increased IgG antibodies -This is common in the blood and plays a key role in creating memory cells that fight viruses-30 to 40 times higher than the control group that received only one dose of AstraZeneca.

The results of the Spanish study were announced at a press conference on May 18, and it was also found that the presence of neutralizing antibodies after Pfizer dose increased sevenfold-significantly higher than after the second AstraZeneca injection.

“If you look at the actual immune response, the Spanish trial’s research shows that if you measure the antibody response of people who take AstraZeneca and then take the second dose of Pfizer, they will have a very strong immune response,” Tam said. “So the promotion response you want to see is very present.”

The Spanish study also found that only 1.7% of participants reported serious side effects, limited to headaches, muscle pain, and general malaise.

The early results of another recent study in the United Kingdom also provided information for NACI’s guidance. The study found that it is possible to mix different COVID-19 vaccines for a second dose. Increase the chance that someone will have a mild or moderate reaction Like fatigue, headache or fever-these symptoms may indicate a strong immune response.

The results are Released on May 12 As a correspondence in the “Lancet” medical journal, the Com-Cov vaccine trial from Oxford Vaccine Group is studying the use of different combinations of the first and second doses of approved COVID-19 vaccines.