This section below lists and describes scientific evidence and jurisdictional experiences regarding SARS-CoV-2 transmission to others following vaccination among health care workers (HCWs) and the general population.

The following limitations should be noted:

• Limited information was identified on the topics of interest. Moreover, there are limited studies that “directly” evaluate transmission, and studies may use different proxy measures to evaluate transmission (e.g., viral load, infection rate). Heterogeneity across study designs, which may also impact study comparisons, could be attributed to follow-up duration, frequency of testing, comorbidity adjustments, underlying seroprevalence, household size, failure to report vaccine status of household contacts, transmission mitigation strategies after index case diagnosis, missing data, and other factors.
• The methodological quality of most of the sources identified are unclear as the Research, Analysis, and Evaluation Branch does not have the expertise to make such assessments; methodological assessments published by other research groups are reported where available. 

Please refer to the following previously completed Evidence Synthesis Briefing Notes for other relevant information on the topics of interest: 

• 67. Evidence Synthesis Briefing Note on COVID-19 Immunization Policies for Hospitals and Health Care Workers (June 8, 2021)
• 71. Evidence Synthesis Briefing Note on Mandatory COVID-19 Vaccination Policies for Health Care Workers (August 13, 2021)

Scientific Evidence

• Most of the identified studies regarding transmission of SARS-CoV-2 among vaccinated (partially or fully) HCWs or the general population suggest that those who do become infected are less likely to pass the virus on to contacts than those who are unvaccinated, thus helping to control outbreaks. Some studies indicate there may be a higher risk of transmission with the Delta variant compared to other variants of concern (VOC). However, there is heterogeneity across the study designs, which may impact the overall findings. For example:
  • A review (Aug 9, 2021) noted that although vaccination has been reported to reduce symptomatic COVID-19 cases, the direct evidence for vaccine-reduced transmission is limited. Reduced viral load has been observed in individuals vaccinated with the Pfizer vaccine, and as lower viral load has been associated with a reduction in onward transmission, these data together suggest that vaccination could reduce transmission.
  • The COVID-19 Living Evidence Synthesis (last updated Aug 25, 2021), published by the Coronavirus Variants Rapid Response Network and the COVID-19 Evidence Network to support Decision-making (COVID-END) in Canada, examines the efficacy and effectiveness of available COVID-19 vaccines for VOC. In terms of transmission outcomes, the review indicated:
    • Household of Vaccinated Individual/HCW (VOC Alpha): Two studies from England and Scotland showed that Pfizer and AstraZeneca vaccines reduced transmission of VOC Alpha from a vaccinated index case (14 to 21 days after the first dose) to household contacts compared to households of unvaccinated index cases (range of mean estimates across the studies: 30-49% from infection). A Finnish study found that Pfizer and Moderna vaccines reduced transmission of VOC Alpha from a vaccinated HCW (10 weeks after the first dose) to a household spouse (42.9% [95% CI, 22.3 to 58.1] from infection).
    • Vaccinated Close Contacts of COVID+ (VOC Alpha): Two studies showed that the Pfizer vaccine reduced transmission to close contacts of COVID+ index cases at least seven to 14 days after the second dose (range of mean estimates across the studies: 65-80% from infection; 94% [95% CI, 60 to 99] from hospitalization). One of these studies also showed that the AstraZeneca vaccine reduced transmission to close contacts of COVID+ index cases at least 14 days after the first dose (44% [95% CI, 31 to 54] from infection and 92% [95% CI, 46 to 99] from hospitalization).
    • Vaccinated HCW versus Unvaccinated Community (VOC Beta and Gamma): A Canadian study found that the Pfizer vaccine reduced transmission of VOC Beta or Gamma from vaccinated HCWs compared to unvaccinated community members ≥14 days after the first dose (54.7% [95% CI, 44.8 to 62.9] from infection) and ≥ seven days after the second dose (84.8% [95% CI, 75.2 to 90.7] from infection).
  • A Public Health Ontario evidence brief on risk of COVID-19 transmission from vaccinated cases (June 2021) reported that lower viral loads and reduced duration of infectiousness are observed in vaccinated individuals infected with SARS-CoV-2. Limited evidence suggests a reduced risk of transmission to household members from infected vaccinated HCWs, including infection from VOC Alpha and Beta.
  • Two studies on HCWs raised concerns about limited protection offered by available vaccines on COVID-19 transmission. One study reported on the case of a breakthrough infection of a fully vaccinated HCW with onward transmission to an unvaccinated partner, and another study found a similar viral load in vaccinated and non-vaccinated HCWs infected by VOC Alpha.

International Scan

• The Public Health Ontario evidence brief (June 2021) on risk of COVID-19 transmission from vaccinated cases reported that the United States, European Center for Disease Control and Prevention (ECDC), United Kingdom, and Australia have maintained public health isolation requirements for fully vaccinated cases in the general population, likely due to the uncertainty around the vaccine effectiveness in sub-populations (e.g., elderly, immune-suppressed) and against the emerging VOC with immune escape potential. No change in the management of contacts of these breakthrough cases was also noted. The evidence brief also highlighted that:
  • The ECDC’s interim guidance on the benefits of full vaccination against COVID-19 for transmission (Apr 21, 2021) has taken a risk assessment approach through synthesizing the evidence to date. They concluded that the likelihood that a fully vaccinated person will transmit SARS-CoV-2 to a unvaccinated individual is very low to low and that the impact of the unvaccinated contact developing severe disease if transmission has occurred is low to high, depending on their age and underlying medical conditions. Other modulating factors may affect the risk of transmission, such as presence of VOC, the nature and duration of contact, the use of prevention measures, the type of vaccine received, and the length of time since vaccination (as duration of immunity following vaccination is not known to date). The ECDC also concluded that the risk of infection and onward transmission of SARS-COV-2 in fully vaccinated individuals should not be considered in isolation, but should always be assessed in the broader epidemiological context of SARS-CoV-2.
  • Germany was the only jurisdiction identified with updated management guidance for fully vaccinated cases and has reduced their isolation time to five days if a negative test result is subsequently obtained and the case remains asymptomatic. Otherwise, if the PCR follow-up test result remained positive or if the individual became symptomatic, the routine isolation requirements apply.

Canadian Scan

• The Public Health Ontario evidence brief on risk of COVID-19 transmission from vaccinated cases (June 2021) reported that Alberta, British Columbia, Manitoba, and Quebec have maintained public health isolation requirements for fully vaccinated cases, likely due to the uncertainty around the vaccine effectiveness in sub-populations (e.g., elderly, immunosuppressed) and against the emerging VOC with immune escape potential. No change in the management of contacts of these breakthrough cases has also been noted.

Ontario Scan

• According to the Public Health Ontario evidence brief on risk of COVID-19 transmission from vaccinated cases (June 2021):
  • Most fully vaccinated individuals with high risk exposures are advised to be tested, but in general, do not have to quarantine if they remain asymptomatic after exposure. This guidance is based on current evidence of vaccine effectiveness and lower risk of secondary transmission if an exposed vaccinated individual was to become infected. However, all confirmed COVID-19 cases, regardless of vaccination status, must isolate for at least 10 days from symptom onset (or 10 days from positive test collection date if they never had symptoms), provided that the individual is afebrile (without the use of fever-reducing medications) and symptoms are improving for at least 24 hours, unless they have severe disease or severe immune compromise. Therefore, current guidance does not account for the potential for lower risk of secondary transmission from vaccinated cases (particularly from asymptomatic cases), the potential for shorter duration of infectiousness in vaccinated cases, or the implication of a negative test after a positive result in a vaccinated case.
  • The current Management of Cases and Contacts of COVID-19 in Ontario (Version 12.0) allows for local public health units to discontinue case and contact management if they assess a low likelihood of infectiousness from a positive result (e.g., in an asymptomatic individual with low pre-test probability, regardless of vaccination status, and a high cycle threshold value with repeat negative test result). Vaccination status may be another factor in assessing a “low pre-test probability” for such cases, given the lower likelihood of infection despite exposure with vaccination, and may support discontinuation of case and contact management.

The COVID-19 Evidence Synthesis Network is comprised of groups specializing in evidence synthesis and knowledge translation. The group has committed to provide their expertise to provide high-quality, relevant, and timely synthesized research evidence about COVID-19 to inform decision makers as the pandemic continues. The following member of the Network provided an evidence synthesis product that was used to develop this Evidence Synthesis Briefing Note:

• Iorio, A., Little, J., Linkins, L., Abdelkader, W., Bennett, D., & Lavis, J.N. COVID-19 living evidence synthesis #6 (version 6.17): What is the efficacy and effectiveness of available COVID-19 vaccines in general and specifically for variants of concern?Hamilton: Health Information Research Unit, 25 August 2021.