This evidence synthesis has been compiled by members of the CITF Secretariat and does not necessarily represent the views of all CITF members.

By Mariana Bego

On December 8th 2020 at 6:30 AM, the first COVID-19 vaccine was administered to a 90-year-old British woman in the United Kingdom. Six months later, over 2 billion doses have been administered worldwide. In this context, the scientific journal Nature explores key vaccine questions as countries race to vaccinate their citizens as viral variants spread around the globe.

It will be years before we truly understand how COVID-19 vaccines have shaped the course of the COVID-19 pandemic, a pandemic that has already claimed over 3.5 million lives. Globally, 2.39 billion doses of the COVID-19 vaccines were administered as of June 13, 2021. In Canada, 61.34% of the total population (12 and older), already received at least one dose of the vaccine, according to the Government of Canada ‘s latest update on June 11, 2021, with data up to June 5, 2021.

The vaccine results have been promising, but certain questions remain unanswered. In a recent Nature article, author Heidi Ledford answers important questions about what we know so far:

How well do the vaccines work in the real world? Several clinical trials show that COVID-19 vaccines can be up to 95% effective in protecting recipients from symptomatic COVID-19. But the effectiveness can be considerably lower among adults over the age of 70.

How effective are the vaccines against variants of concern? Research to date shows significant protection, although a small proportion of people can still get infected with variant strains of the virus, even after two doses of a vaccine. This protection against variants does vary from vaccine to vaccine and from variant to variant. For example, several vaccines showed lower effectiveness (e.g., 51% for Novavax, and 65% for Johnson & Johnson) at preventing symptomatic COVID-19 when faced with the Beta variant (first detected in South Africa, B.1.351).

How long does vaccine protection last? Long-term data have started to emerge from clinical-trial participants who had their first doses in July 2020. With the data to date, it is still unclear if or when immune responses will wane over time; vaccine developers are now testing variant-specific boosters in preparation for this possibility.

Can vaccines block transmission of the virus? Preliminary reports indicate that vaccines can be 50-75% effective against asymptomatic infections. Initial data also suggests that vaccinated people who do not have symptoms are not infecting others, so vaccines are indeed likely blocking transmission.

What do we know about the safety of COVID-19 vaccines? While no vaccine is completely free of side effects, the safety of COVID-19 vaccines is remarkable, with most people experiencing minor side effects such as headaches and fatigue. The author also describes the rare adverse side effects recorded to date.

What impact have the vaccines had on the course of the pandemic? Countries which conducted their vaccination campaigns while implementing social-distancing measures, like Israel and the UK, have seen considerable declines in deaths and hospitalizations from COVID-19. Others, like Chile, have embarked on a vaccination campaign but have rolled back distancing requirements too quickly, resulting in increases of COVID-19 patients. Canada was not mentioned in this article, although it, like Israel and the UK, conducted their vaccine roll-out while continuing social distancing measures.

In summary, we are still learning how well vaccines control the spread of the virus, with promising early data suggesting that vaccinated people who do not have symptoms are not infecting others and are therefore not transmitting the virus. It is still not clear how many people have to be vaccinated against COVID-19 before the population can be considered protected, especially in the context of new variants. And finally, until we reach community-level protection by increasing vaccination, masks and social distancing are still necessary to control this pandemic.


Ledford, H. Six months of COVID vaccines: what 1.7 billion doses have taught scientists. Nature 594, 164-167. 2021 Jun 4. doi: 10.1038/d41586-021-01505-x.