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Question: Are we heading toward a seasonal COVID-19 vaccination like the flu shot?

Answer: At least for this fall, it looks like we will have an updated vaccine that is expected to provide some protection against recent variants of SARS-CoV-2, the virus that causes COVID-19.

Last week, Moderna announced it is developing a “bivalent booster,” targeting both the original version of the virus and the highly contagious Omicron variant, which has caused a surge in COVID-19 cases around the world in recent months.

“A bivalent booster may provide broad, durable protection against several different variants, even ones they weren’t specifically designed to target,” Shehzad Iqbal, medical director of Moderna Canada, said in an e-mail.

He added that the company is providing clinical data to various government regulators, including Health Canada, in the hopes of gaining approval for its new vaccine by the fall.

Another major vaccine manufacturer, Pfizer-BioNTech, is also working on an updated version of its COVID-19 shot. Pfizer did not respond to requests for an interview.

Doctors still advise those due for their booster or fourth shot to get their doses as scheduled and not wait until the fall for the arrival of the bivalent vaccine, because they would likely experience waning protection before then.

Health experts generally agree that there is a need to modify the vaccines so that they are a better match for the virus, which has continued to mutate and evolve into new variants.

Vaccines are designed to train the immune system to be on the lookout for a particular pathogen. In the case of SARS-CoV-2, the vaccines target the spike protein, a knobby protrusion on the surface of the virus. The immune system generates antibodies that can latch onto the spike protein and prevent the virus from invading cells. If the virus gets through this first line of defence, other specialized immune cells will spring into action.

However, SARS-CoV-2 has changed so much that the antibodies have a harder time recognizing and adhering to the spike protein, said Omar Khan, a professor of biomedical engineering and immunology at the University of Toronto.

As a result, people who have received multiple shots of the current vaccines can experience breakthrough infections, although they still retain fairly effective protection against developing severe disease, which can lead to hospitalization and death.

The good news is that the vaccines made by both Moderna and Pfizer-BioNTech are based on easily modifiable messenger RNA – a relatively new approach for creating inoculations.

“You can synthesize your RNA with just basic chemistry,” explained Dr. Khan. “You mix the components together and they self assemble into nice little nanoparticles.”

By comparison, some flu shots are created by first growing influenza strains in eggs. These virus particles are then modified for the vaccine so that the shots trigger an immune response without actually making people sick.

The mRNA vaccines are certainly easier to produce than traditional vaccines. But the companies still need to go through regulatory approval and provide clinical trial data to prove the jabs are safe and effective.

“Clinical trials can take months to complete,” Dr. Khan said.

In time, it’s possible that the COVID-19 vaccine approval process will be streamlined to resemble the regulatory framework used for annual flu shots, which don’t require lengthy clinical trials.

“We understand the flu vaccines really well. And altering the strains that are used in them isn’t going to change their safety profile,” said Matthew Miller, a professor at McMaster University’s Institute for Infectious Disease Research.

“Obviously, regulators are going to do their job to make sure the new vaccines are safe.” But, he added, as familiarity with COVID-19 mRNA vaccines increases, clinical trials may no longer be required for the approval of updated shots.

What is still uncertain is how often the vaccines will need to be modified.

The virus has gone through a lot of mutations partly because it has infected so many people. Every time it invades a body, there is a risk it could mutate into a more contagious variant, Dr. Miller said.

As infections decline, “we should expect that the rapid rate of variant emergence that we have witnessed over the last couple of years will start to slow down,” he added.

In turn, a slower rate of viral evolution should mean that our vaccines remain more effective for a longer period of time, said Dr. Khan.

But even if the virus itself doesn’t change much from year to year, there may still be a good reason to provide regular booster shots, said Rob Kozak, a scientist and clinical microbiologist at Sunnybrook Health Sciences Centre in Toronto.

He noted that the number of COVID-19 antibodies circulating in the bloodstream tends to decline over time following vaccination.

The main job of ongoing COVID-19 surveillance is carried out by memory B cells. If they detect the virus, they can reprogram themselves to start churning out antibodies. But it can take a while for them to ramp up production. In the meantime, the infected person may experience symptoms of the respiratory illness.

A booster shot will raise circulating antibody levels and may help reduce the risk of such infections, said Dr. Kozak.

Another uncertainty is the durability of protection. We simply don’t know how long COVID-19 immunity lasts – regardless of whether it is acquired through a vaccine or an actual infection.

Dr. Kozak points out that SARS-CoV-2 is a coronavirus – the same family of pathogens that cause some forms of the common cold.

“If you get a cold, you seem to be protected maybe for a year or two, and then you can get reinfected with the same virus,” he said. “Your immunity tends to wane.”

Researchers might find that immunity to COVID-19 wanes in a similar fashion.

So, for a variety of reasons, public-health officials may come to the conclusion that COVID-19 vaccine campaigns are needed every year – mirroring the rollout of the annual flu shot.

Administering the vaccines in the fall would raise antibody levels in the community and possibly help blunt a wave of COVID-19 cases in winter months when people tend to congregate indoors – a behaviour that favours the spread of respiratory illnesses.

Bringing down overall infections should also reduce transmission of the virus to vulnerable people who are more likely to develop severe disease, such as the immunocompromised and older adults, Dr. Kozak said.

“Boosters can perform a broader public-health good, providing benefit beyond those who get the shots.”

Paul Taylor is a former Patient Navigation Adviser at Sunnybrook Health Sciences Centre and former health editor of The Globe and Mail.