Overview

• A large body of evidence suggests that vaccines are safe and effective against COVID-19.
  
• All viruses evolve over time through genetic variations. These changes can cause the emergence of new “variants” of the original virus and may have different properties than the original virus, such as increased transmissibility or the ability to cause more severe disease. Omicron, along with its sub-variants, is the currently circulating variant of concern (VOC) in Alberta.
  
• Omicron has demonstrated increased transmissibility and capacity to evade the body’s immune response compared to previous versions of the virus. This occurs whether this immunity was generated by infection, vaccines, or both.
  
• The protection that vaccines provide against severe disease continues to be strong even many months after immunization. Two doses of a COVID vaccine dramatically reduce the risk of hospitalization or death, and a third dose will offer even more protection. A booster dose is recommended at 5 months after the second dose, and at least 3 months following infection with COVID-19, to increase the level of protection against severe outcomes.
  
• COVID vaccines are not always able to provide protection against infection by Omicron or one of its subvariants. Immunity against infection wanes over time, but can be boosted with an another dose of an mRNA vaccine.
  
• Booster doses are most important for those at high risk of severe outcomes. For considerations on these doses, see https://www.alberta.ca/covid19-vaccine.aspx

The Evidence

A systematic review of the evidence published between December 2021 and April 2022 in The Lancet Infectious Diseases found that the VE of a primary series (for most people, this is two doses) against severe outcomes remained high and stable over time at over 80%. ^{[1, 2]} VE against infection by Omicron was lower than for previous variants and waned over time with most protection against infection lost by 4-6 months post second dose. However, boosting with a third dose increased the VE against infection with Omicron (to about 60%) and against severe outcomes (to 90%). ^{[1, 2]}

The figure on page 20 of this document helps to illustrate the results from the evidence, for both the primary series as well as for a booster dose.
Another evidence review, published by McMaster University on June 22, 2022, also examined the effectiveness of COVID-19 vaccines against the Omicron variant. It found that in general, people with a primary series continue to have a good level of protection against hospitalisation and death due to COVID-19 for 7 to 8 months after being vaccinated. A third (booster) dose increases this protection against COVID-19 related hospitalisations, with a very minimal degree of waning after several months. ^[3]

In addition to reviews that assess the VE of second and third doses, there is emerging evidence on the benefit of a fourth dose. A recently published study in healthcare workers from Israel found that only 7% of those with a fourth dose became infected with Omicron, versus 20% of those with only three doses, demonstrating increased protection against infection. Infections were either asymptomatic or mild and no severe outcomes were seen. ^[4]

Note that this data is current up to August 5, 2022 and will be updated periodically as new information emerges.

References

1. Higdon, MM, et al. 2022. Duration of effectiveness of vaccination against COVID-19 caused by the omicron variant. The Lancet Infectious Diseases. Published online June 22, 2022. Accessed at: https://doi.org/10.1016/S1473-3099(22)00409-1

2. Higdon MM, Baidya A, Walter KK, et al. Supplement to: Duration of effectiveness of vaccination against COVID-19 caused by the omicron variant. The Lancet Infectious Diseases. Published online June 22. Accessed at https://www.thelancet.com/cms/10.1016/S1473-3099(22)00409-1/attachment/3ba9ad38-48b8-46a3-a993-baddc6bf8bbd/mmc1.pdf

3. Bacon, SL, et al. 2022. COVID-19 living evidence synthesis #10 (version 10.7): What is the long-term effectiveness of available COVID-19 vaccines for adults, including for variants of concern and over time frames beyond 112 days in those with a primary series and beyond 84 days in those with a primary series and an additional dose? The Montreal Behavioural Medicine Centre, META group. Published online June 22, 2022. Accessed at: https://www.mcmasterforum.org/docs/default-source/product-documents/living-evidence-syntheses/covid-19-living-evidence-synthesis-10.7---what-is-the-long-term-effectiveness-of-available-covid-19-vaccines-for-adults.pdf?sfvrsn=4c92f905_5

4. Cohen MJ et al. 2022. Association of receiving a fourth dose of the BNT162b Vaccine with SARS-CoV-2 infection among health care workers in Israel. JAMA Netw Open. 5(8):e2224657. Published online August 2, 2022. Accessed at: https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2794864

Further reading

1. National Advisory Committee on Immunization (NACI)
   https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci.html
   
2. McMaster Forum COVD-19 Evidence Network to support Decision Making
   https://www.mcmasterforum.org/networks/covid-end/resources-to-support-the-public
   
3. United States Center of Disease Control (CDC)
   https://www.cdc.gov/media/releases/2022/s0715-COVID-VE.html