Two (2) doses of Moderna Spikevax XBB.1.5 or 3 doses of Pfizer-BioNTech Comirnaty XBB.1.5 are recommended, with an 8-week interval between doses. An additional dose is recommended for individuals who are moderately to severely immunocompromised, with an interval of 4 to 8 weeks between each dose (see Table 1 for schedules and dosages).

The 8-week interval is longer than the authorized interval and is recommended as a better immune response has been associated with a longer interval between doses. For those who are moderately to severely immunocompromised, the 4 to 8-week interval between doses needs to balance a potentially better immune response with longer intervals with the need for earlier protection due to risk of exposure from circulating SARS-CoV-2 and the risk of severe COVID-19 disease.

Those who are moderately to severely immunocompromised are recommended to receive an additional dose in the primary series compared to those who are not immunocompromised in order to help improve the immune response and vaccine effectiveness in this group (based on data from adults). Those who are moderately to severely immunocompromised generally respond less well to COVID-19 vaccines and are at higher risk for severe illness.

If children 6 months to under 5 years of age started the primary series with a non-XBB.1.5 vaccine (i.e., original monovalent, BA.1 bivalent or BA.4/5 bivalent) but did not complete the series, they should complete the primary series with an XBB.1.5 vaccine (see Table 2).

For children 6 months to under 5 years of age who are moderately to severely immunocompromised, a 3-dose series of the Moderna Spikevax (25 mcg) vaccine is preferred over 4 doses of Pfizer-BioNTech Comirnaty (3 mcg) because there is likely higher acceptability and more feasible implementation due to fewer doses in the schedule using Moderna Spikevax.

A mixed product schedule using vaccines from different manufacturers can be offered for the primary series, however if both Pfizer-BioNTech Comirnaty and Moderna Spikevax vaccine products are used in the same primary series for an individual 6 months to under 5 years of age, the total number of doses in the series should follow the Pfizer-BioNTech Comirnaty schedule (see Table 2).

Children who started the primary series with a non-XBB.1.5 vaccine at less than 5 years of age and turn 5 years of age before completing the series, should receive the number of XBB.1.5 vaccine doses recommended as per Table 2 for those 5 years of age and older (including relevant footnotes). Children who started a primary series with an XBB.1.5 vaccine at less than 5 years of age and turn 5 years of age before completing the primary series, should receive the following with the age-appropriate dosage for their current age:

• the total number of XBB.1.5 doses for a primary series for those less than 5 years of age (see Table 1 for number of doses and intervals); AND
• the number of doses they receive after turning 5 years of age should not exceed the number of doses for someone commencing the primary series at 5 years of age or older (i.e., the number of doses should not exceed 1 dose for those not immunocompromised, and 2 doses for those who are moderately to severely immunocompromised).

One dose of XBB.1.5 vaccine is recommended as per the authorized schedule in the product monograph. An additional dose is recommended for individuals who are moderately to severely immunocompromised, with an interval of 4 to 8 weeks between the two doses (see Table 1 for schedules and dosages).

Those who are moderately to severely immunocompromised are recommended to receive an additional dose in the primary series compared to those who are not immunocompromised in order to help improve the immune response and vaccine effectiveness in this group. Those who are moderately to severely immunocompromised generally respond less well to COVID-19 vaccines and are at higher risk for severe illness.

The 4 to 8-week interval for those who are moderately to severely immunocompromised needs to balance a potentially better immune response with the longer interval with the need for earlier protection due to risk of exposure from circulating SARS-CoV-2 and the risk of severe disease. As well, the risk of myocarditis/pericarditis following the second dose has been determined to be lower with longer intervals between doses.

If a person 5 years of age and over started the primary series with a non-XBB.1.5 vaccine (i.e., original monovalent, BA.1 bivalent or BA.4/5 bivalent) but did not complete the series, they should complete the primary series with an XBB.1.5 vaccine based on the total number doses previously recommended (i.e., they should receive a total of 2 COVID-19 vaccine doses in the primary series if not immunocompromised and 3 doses in the primary series if they are immunocompromised) (see Table 2).

For children who started a primary series at less than 5 years of age and turn 5 years of age before completing the primary series, see guidance above for Children 6 months to under 5 years of age.

There is no longer a product preference between Moderna Spikevax and Pfizer-BioNTech Comirnaty with the use of XBB.1.5 COVID-19 vaccines for unvaccinated individuals 12 to 29 years of age. Previously, Pfizer-BioNTech Comirnaty had been preferred over Moderna Spikevax for the primary series among individuals 12 and 29 years of age due to the higher risk of myocarditis/pericarditis observed following the Moderna Spikevax 100 mcg original monovalent vaccine primary series (especially after the second dose). This product preference has now been removed. As compared to the original monovalent primary series, the risk of myocarditis/pericarditis is now expected to be lower due to the use of a 1-dose schedule in most individuals and potentially due to a lower dosage of the available Moderna Spikevax vaccine (50 mcg in the XBB.1.5 formulation compared to 100 mcg in the original monovalent formulation).

mRNA vaccines are the preferred products. It is recommended that a primary series of the authorized protein subunit COVID-19 vaccine (Novavax Nuvaxovid XBB.1.5) should be offered to those 12 year of age and older who are not able or willing to receive an mRNA COVID-19 vaccine. Preference of mRNA vaccines over the protein subunit vaccine is due to the availability of more data with regard to the benefits and risks of mRNA vaccines compared to the protein subunit vaccine. Recommendations regarding the use of the protein subunit vaccine are currently under review by NACI. Both the original mRNA vaccines and Novavax Nuvaxovid original have been associated with a rare risk of myocarditis/pericarditis. Data is not yet available for XBB.1.5 vaccines.

When offering Novavax Nuvaxovid XBB.1.5 as a primary series to those 12 years of age and older who are moderately to severely immunocompromised, it should be noted that the safety and efficacy has not been established in these individuals. Informed consent for use of Novavax Nuvaxovid XBB.1.5 for those who are moderately to severely immunocompromised should include the above information, and the well- documented evidence on the safety profile and effectiveness of mRNA COVID-19 vaccines in these populations based on real world use with large numbers of individuals.