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Dr. Zain Chagla is an associate professor at McMaster University. Dr. Isaac Bogoch is an associate professor at the University of Toronto. Dr. Sumon Chakrabarti is a lecturer at the University of Toronto.

Due to groundbreaking advances in vaccine development, there are many continuing advanced human clinical trials of COVID-19 vaccine candidates. The narrative has been consistent: vaccination will facilitate our path back to normalcy. Vaccine development milestones now headline the news daily, and the population is more invested than ever before.

However, it is important to consider that a vaccine may not automatically equal the end of COVID-19. Vaccination goes back centuries, marking one of the most significant interventions in medicine. Vaccine efficacy lies on a spectrum at the population and individual level. Some vaccines prevent nearly all individuals from getting infected, whereas others may reduce but not eliminate the risk of getting infected, or may reduce the disease’s severity.

So what are the potential scenarios for the COVID-19 vaccine?

One scenario is the rollout of a vaccine that prevents nearly all person-to-person spread. This is clearly the best-case scenario and in essence would effectively halt the pandemic, allowing us to return to our previous activities, through herd immunity.

There are diseases where vaccines prevent virtually all transmission, such as Hepatitis A and B, measles, and polio. Eradication is possible for these diseases, but has not been realized due to implementation issues, vaccine access, poor immune responses in some, and vaccine hesitancy (when people delay or refuse to get vaccinated).

A second scenario is the rollout of a vaccine that prevents some degree of person-to-person spread, and reduces death and severe illness. This is the most likely scenario – a vaccine that prevents some but not all person-to-person SARS-CoV-2 transmission. This still has large implications for our health systems and personal safety, taking us along the path to normalcy.

The influenza vaccine would be the closest comparison in this regard. It reduces the severity of influenza in those who take it, but does not completely prevent illness in all. With milder symptoms, people are less contagious, which in turn results in an overall decrease in transmission.

High rates of immunization are needed for maximum effectiveness. Influenza vaccine uptake has historically been abysmal despite the evidence pointing toward its benefit. Only 42 per cent of Canadians received the influenza vaccination in 2018-2019, with rates among health care workers only slightly higher. The uptake is hurt by vaccine hesitancy and the perceived imperfection of the vaccine.

As the fall season comes, the seasonal influenza vaccine campaign has more of an impetus for maximizing immunization rates. Given the health care burden, as well as the clinical overlap created with COVID-19 and influenza transmission in the community, it is imperative that influenza vaccine rates reach peak potential. Furthermore, this is a unique “proof of principle” concept that can show that a vaccine rollout to the general population can attain high levels of community uptake. It can also be used to troubleshoot for an effective COVID-19 vaccine rollout campaign.

A third scenario is the rollout of a vaccine that does not prevent person-to-person spread, but reduces death and severe illness. This scenario is clearly not optimal, but still has potential benefits. For example, the vaccine for tuberculosis does not reduce the risk of acquiring it, but limits complications, particularly in the young.

If the SARS-CoV-2 vaccine were similar, eliminating the spread would be unlikely. However, relief from the pandemic by dampening outbreaks and reducing complications is still progress. The hope is that with further therapeutic advances, this will become a manageable illness.

We shouldn’t forget one final scenario – that the vaccine won’t work. But there is hope that another vaccine in development would then fill this role.

Proactive messaging is key for the best chance of achieving high vaccine uptake globally. We also need to set realistic expectations for the possibility that vaccination does not eliminate COVID-19. Physical distancing, hand hygiene, improved ventilation of indoor settings, and indoor mask use are now a part of our daily routines, and will be with us for some time. This is the new normal and we should continue living by these principles to keep community rates as low as possible. This all said, the staggering degree of human ingenuity driving vaccine development is a shining beacon of light at the end of the tunnel.

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