Table of contents

Posted by

Dr. Deena Hinshaw


July 20, 2022



With expanded eligibility for second boosters now in place in Alberta, here are some factors to consider when deciding whether to get a second booster and when to get it.

It is important to look at what the current vaccines offer with Omicron sub-variants as our dominant strains. Before Omicron, vaccines were highly effective at preventing infection, and even better at preventing severe outcomes.1 Since Omicron began to spread widely, its mutations have meant that vaccine doses are less able to prevent infection in a long term way.2 The good news is that vaccines work very well to reduce the risk of severe outcomes and this protection lasts a long time.2 It’s still very important to get the primary series and first booster. These provide the biggest improvement in protection, and for anyone who hasn’t yet had their first, second or third dose, now is a good time to get that extra protection.

Bivalent vaccines

Different kinds of vaccines are being developed, with a possibility that we will have a bivalent vaccine that includes an Omicron strain in the fall, but availability is, at minimum, months away. Research on the bivalent vaccine has mostly been done with a BA.1 component, and early results seem to show better antibody response against Omicron infections when compared with the original vaccine.3 It is not clear yet what level of benefit will be seen with any future variants.

We are left with decisions about what we do now with the vaccines we currently have, given their main current impact is against severe outcomes. We know that for most people, severe outcome protection after the first booster stays very high for at least 4 to 6 months, but there can begin to be some reduction in this protection over longer periods of time.4

Hybrid immunity

Another thing to keep in mind is how vaccines and COVID-19 interact. People who have had an infection have an immune response afterwards, but this response, especially after Omicron infections, provides even more protection in those who are also immunized.5 This combination of vaccine protection (ideally an initial series plus a booster) and infection results in “hybrid immunity”, so it’s recommended to wait at least 3 months after infection before getting a booster dose.

Vaccine intervals

Another factor to consider is that while there is clear short-term benefit to an additional dose, we still need more research to understand the long-term impacts of receiving multiple doses of the same formulation of COVID-19 vaccines within a relatively short time, especially at short intervals. What we do know is that a longer time between vaccine doses is better for optimal response, 6, 7 which means those who take a booster now may have to wait longer for access to a fall booster.

Exposure risk

Finally, an important consideration is the timing of exposure risk. Knowing that the greatest protection after a vaccine is experienced 2 weeks to several months after receiving it, it makes the most sense to offer it shortly before a time of high exposure risk.

This is why our annual influenza vaccine campaign happens in October – because the highest transmission time for influenza is typically November through January. The same considerations hold true for COVID-19, although for the past 2 years we have had high transmission throughout the year in multiple waves, making the timing of vaccine doses difficult to plan.

We are currently experiencing a rise in transmission driven primarily by the BA.5 variant. It is likely we will have a resulting small to moderate wave in the next few months, and a larger wave in later fall and winter. Transmission risk will be high at both times, but likely higher in the colder season.

Choosing a second booster dose

So where does this leave us with respect to vaccine decisions? Alberta’s eligibility criteria are intended to facilitate choice. Not everyone needs a second booster at this time, and it is important for people to weigh their options in light of their own specific situation.

Current evidence shows that a primary series (for most people this is two doses) plus a booster is highly effective in preventing severe outcomes in the majority of people, especially for those who are younger than 50 and have no medical risk factors. If you are in this category, it may be best to wait for the fall and take a second booster then.

For anyone who has had all previously recommended doses and has had COVID-19 infection in the past 3 months, even if you are older or have medical risk factors, it may also be best to wait for the fall and take a second booster then.

Congregate Care and health-care workers

For those who have risk factors such as living in seniors’ congregate care settings or age over 50, we generally recommend a second booster, but this is a risk-benefit decision. If it has been more than 5 months since a previous dose, and you are planning activities this summer that will lead to higher numbers of interactions with people outside your household, it may be best to get a second booster now as long as you have not had COVID-19 in the last 3 months.

On the other hand, waiting to the fall has the advantage of possibly being able to access a different type of vaccine. If you are able to take other precautions this summer, you may want to wait. Similar risk-benefit considerations would apply to those with medical risk factors under the age of 50.

For healthcare workers who may be considering getting a second booster of vaccine to protect those around them, there would be a short-term benefit in this dose. This should be weighed against the fact that the timing of access to a fall dose could be impacted by the timing of this dose and the anticipation that we may have higher transmission in the winter than what we will see in the coming few months.

Other prevention options for high-risk individuals

Whatever vaccine decisions people make, it is also important to remember that there are also early treatment and prevention options for those at the highest risk of severe outcomes. For example, Evusheld™ is a type of 'borrowed immunity' that can give extra protection for about 6 months to immunocompromised individuals who may not be able to fully generate their own antibodies against COVID-19 after vaccination or infection. Evusheld™ can be given to those who have had COVID-19 vaccination and can also be used in those for whom vaccines are contraindicated. Albertans wishing to access Evusheld™ are asked to speak with their doctor about whether they may be eligible.


More than 2 years into the pandemic, it can be frustrating to not have a simple answer on how best to protect ourselves and our communities, but the reality is that we are still learning. Expanding access to an additional booster right now is a response to the BA.5 wave, for the purpose of allowing choices about enhancing individual protection against severe outcomes. Those at the highest risk of severe outcomes who have not had COVID-19 in the last 3 months will benefit most from getting a second booster now. This change is being made transparently, sharing what we know and what we don’t know, so that people can make decisions about the best action for themselves.


  1. Duration of effectiveness of vaccines against SARS-CoV-2 infection and COVID-19 disease: results of a systematic review and meta-regression
  2. Duration of effectiveness of vaccination against COVID-19 caused by the omicron variant
  3. A Bivalent Omicron-containing Booster Vaccine Against Covid-19
  4. Waning 2-Dose and 3-Dose Effectiveness of mRNA Vaccines Against COVID-19-Associated Emergency Department and Urgent Care Encounters and Hospitalizations Among Adults During Periods of Delta and Omicron Variant Predominance - VISION Network, 10 States, August 2021 - January 2022
  5. Protection against Omicron BA.2 reinfection conferred by primary Omicron or pre-Omicron infection with and without mRNA vaccination
  6. Serological responses and vaccine effectiveness for extended COVID-19 vaccine schedules in England
  7. Two-dose SARS-CoV-2 vaccine effectiveness with mixed schedules and extended dosing intervals: test-negative design studies from British Columbia and Quebec, Canada
  • Dr. Deena Hinshaw

    Dr. Deena Hinshaw

    Dr. Deena Hinshaw was appointed Alberta's Chief Medical Officer of Health on January 28, 2019.

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