VACCINE RESOURCE COMMITTEE

of the Edmonton Zone Medical Staff Association

View Full List of Vaccine Questions Here

COVID-19 Vaccine Question #2:

Why do people need more than one COVID-19 or influenza vaccine?

Viruses like influenza and SARS-CoV-2 mutate over time but modifications to the vaccines in order to match the changed viruses help to keep immunity current to match the circulating virus strains. In addition, protection from some vaccines wane over time, so a repeat vaccine dose to boost the immunity can help.

The virus that causes COVID-19 continues to change; therefore, vaccines are updated periodically to help fight the disease. The vaccine is updated to better protect against variants that are currently spreading. 

Research is continuously evaluating effectiveness. The vaccine has been updated as the virus has mutated. Vaccine effectiveness of XBB.1.5 mRNA COVID-19 vaccines in adults has been estimated to be approximately 50 to 60% against symptomatic disease (on top of the protection from prior vaccines) and 60 to 70% against hospitalization. Vaccin effectiveness to infection and severe infection had decreased due to waning immunity and a changing virus. Protection is higher in those with previous infection and vaccination (hybrid immunity) than in those who have only been vaccinated or only been infected. Vaccination prevents transmission as those who are not infected cannot spread infection to others, and some studies show that vaccinated people with breakthrough infections are less likely to spread infection in their household.

The 2024–25 COVID vaccine was effective for recent viral strains, it is recommended that everyone stay up to date by getting this vaccine. Previous vaccines or boosters were not developed to target the more recent strains.

It usually takes about two weeks for the vaccine to become effective. Studies suggest that COVID-19 vaccines are most effective during the first three months after vaccination.

Over the past decade, the influenza vaccine effectiveness has ranged between 40-70% year after year. Alberta contributes data to the SPSN network that produces effectiveness reports at mid-season and end of season every year. In 2024-25 at the mid-point, the vaccine was estimated to be about 50% effective against influenza.

Sources:

Eurosurveillance: Interim estimates of vaccine effectiveness against influenza A(H1N1)pdm09 and A(H3N2) during a delayed influenza season, Canada, 2024/25
John Hopkins Medicine-COVID-19 Vaccine: What You Need to Know