COVID-19 Post-Vaccine Risks, Activities, and Public Health Measures

Last Updated: May 12, 2021

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This Briefing Note was completed by the Research, Analysis, and Evaluation Branch (Ministry of Health). Please refer to the Methods section for further information.


This briefing note summarizes the scientific evidence on the estimated risks of infection, transmission, and other health-related outcomes following full or partial COVID-19 vaccination of various populations. It also includes jurisdictional guidance on recommended activities post-vaccination, including public health measures and travel.

*The full version of the Briefing Note including the Appendix can be accessed in the PDF file at the top of the page*

Key Findings

Low quality research evidence on the risk of COVID-19 infection and other health outcomes after full or partial vaccination was identified from three reviews and 44 single studies (19 of these are preprints) from nine jurisdictions: Denmark, Europe, Germany, Israel, Italy, Qatar, Spain, United Kingdom (UK), and the United States (US). Information related to the risk of infection post-vaccination consists of:

Analysis for Ontario

As recommended by the Public Health Agency of Canada, if 75% of eligible population has received the first dose, and 20% have the second dose, some public health measures may be relaxed while following local public health advice (i.e., physical distancing and mask wearing).

Implementation Implications

Post-vaccine surveillance is recommended including testing and contact tracing for anyone vaccinated who has been exposed.

Supporting Evidence

This section below summarizes the scientific evidence and jurisdictional information on the risk of infection and recommended activities following full or partial vaccination for COVID-19.

Scientific Evidence

Low quality research evidence on the risk of COVID-19 infection and other health outcomes after full or partial vaccination was identified from three reviews and 44 single studies (19 of these are preprints) from nine jurisdictions: Denmark, Europe, Germany, Israel, Italy, Qatar, Spain, the United Kingdom (UK), and the United States (US).

  • Risk of COVID-19 Infection Post-Vaccination:
    • Likelihood of Transmission Post-Vaccination: A review (March 29, 2021) from the European Centre for Disease Prevention and Control (ECDC) identified only one Scottish study with direct evidence of the impact of vaccination on the risk of transmission (as of March 14, 2021). This large study of HCWs suggested that vaccination (at least one dose) of a household member reduces the risk of infection in susceptible household members by at least 30%. Overall, the review recommended follow-up of cohorts with previous SARS-CoV-2 infection and vaccination is needed to better assess the magnitude and duration of protection from reinfection leading to asymptomatic/symptomatic disease, as well as the effect of protection against further transmission to contacts.
      • A large UK study (preprint, April 28, 2021) estimated the likelihood of transmission by individuals, who have received one dose of vaccine but still become infected with SARS-CoV- 2 up to 60 days after the first dose, is 40-50% lower for households in which the index cases are vaccinated 21 days or more prior to testing positive (compared to no vaccination), with similar effects for both Pfizer/BioNTech or AstraZeneca.
    • Reduced Viral Load After Vaccination: The ECDC review (March 29, 2021) noted there is evidence that vaccination significantly reduces viral load and symptomatic/asymptomatic infections in vaccinated individuals, which could translate into the potential for reduced transmission. For example, an Israeli study (March 29, 2021)found that viral load was substantially reduced for infections occurring 12-37 days after the first dose of vaccine.
    • Vaccine Effectiveness (VE) with Partial Vaccinations: After a single dose, eight single studies (Israel, Qatar, UK, and the US) found a high level of protection or reduced risk of infection (Pfizer/BioNTech or AstraZeneca) approximately three to four weeks, which can provide support for national policies of extending the gap between doses up to two or three months (e.g., UK). For example, in Scotland, a population-based study found mass roll-out of the first doses of the Pfizer/BioNTech and AstraZeneca vaccines 28-34 days post-vaccination was associated with substantial reductions in the risk of hospital admission due to COVID-19. Estimated VE for various populations and settings include:
      • HCWs: Numerous single studies estimated decreased risk of infection in HCWs approximately three weeks after a single dose. These findings may support delaying the second dose in countries facing vaccine shortages and scarce resources, thus allowing higher population coverage with a single dose. However, longer follow-ups to assess the long-term effectiveness of a single dose is needed to inform a second dose delay policy. For example: 
        • According to an Israeli study (March 2021), adjusted rate reductions of SARS-CoV-2 infections in HCWs vaccinated with Pfizer were 30% and 75% for days 1-14 and days 15-28 after the first dose, respectively.
        • In the US, two studies (April 2021 and May 2021) on HCWs highlighted the critical importance of continued public health mitigation measures (e.g., masking, physical distancing, daily symptom screening, regular testing), particularly in the early days post-vaccination. Another study (April 2021) found that HCWs infected more than two weeks after first vaccination dose compared to before two weeks from first dose were older and reported fewer symptoms. 
      • LTC: A review of three small studies (May 2021) that reported outbreaks in care homes in the US and Germany following the administration of one to two doses of the Pfizer/BioNTech vaccine highlights the continued risk for this population in the period immediately following the start of vaccination drives in care homes. Administering the full two doses of vaccination should be the priority amongst these vulnerable populations. While another review (April 2021) found evidence of the protective effect of COVID-19 vaccines against infection among people living in care homes (i.e., protection against symptomatic and asymptomatic disease), it is nevertheless advised to maintain non-pharmaceutical measures to protect care home residents until the vaccine has developed its protective effect. A large Danish study (March 2021) also found lower VE for LTC residents between the first and second dose as compared with other populations (e.g., HCWs).
      • Immunosuppressed Populations: Transplant recipients (i.e., liver, heart, and kidney) and people with cancer (i.e., hematology malignancy) developed substantially lower immunological responses to the Pfizer-BioNTech vaccine, suggesting the need to re-evaluate vaccine regimens in these populations. A UK study (April 2021) recommended prioritizing patients with cancer for an early (day 21) second dose of the Pfizer vaccine.
      • Seniors: Two UK studies (March 2021) found that individuals who received a single dose of vaccine (Pfizer/BioNTech or AstraZeneca) resulted in substantial reductions in the risk of COVID-19-related hospitalization in elderly, frail patients.
    • VE with Full Vaccination: Several studies (Scotland, US, and Israel) have reported high VE leading to improved health outcomes after full vaccination, including reduced risk of COVID-19-related hospital admissions, severe illness, and death. For example: 
      • UK: Two UK studies (March 2021 and May 2021) estimated that VE of Pfizer/BioNTech or AstraZeneca vaccines is 67-70% three weeks after the first dose and 85% after the second dose, concluding the vaccine can prevent both symptomatic and asymptomatic infection in working-age adults, including the B.1.1.7 variant.
      • Israel: Three population-based single studies in Israel found high VE after the second dose for reducing serious outcomes in the general population, including hospitalization, severe illness, and death. For example, a study (May 2021) found two doses of Pfizer/BioNTech are highly effective across all age groups in preventing symptomatic and asymptomatic SARS-CoV-2 infections and COVID-19-related hospitalizations, severe disease, and death, including those caused by the B.1.1.7 variant.
      • US: Several US studies also demonstrated the effectiveness of a two-dose regime. For example, a US study (February 18, 2021) demonstrated the real-world effectiveness in reducing the rates of SARS-CoV-2 infection and COVID-19 severity among individuals at highest risk for infection with two COVID-19 vaccine doses in several US states (e.g., Arizona, Florida, Minnesota, Wisconsin).
  •  Single Vaccine Dose for Persons with Confirmed Previous infection: Five small single studies (Italy, UK, and the US) provided some evidence that a single vaccine dose is sufficient in symptomatic SARS-CoV-2-exposed subjects, suggesting no need for a second dose. The UK study (April 2021) recommends that individuals with no history of SARS-CoV-2 infection should be prioritized for a second vaccine inoculation.
  • Effectiveness of National Vaccine Campaigns: In Israel, an analysis (February 24, 2021) revealed that two months after the initiation of the vaccination campaign, with 85% of individuals older than 60 years already vaccinated with two doses, there was an approximately 77% drop in cases, a 45% drop in positive test percentage, a 68% drop in hospitalizations, and a 67% drop in severe hospitalizations. Another population-based Israeli study (March 2021) provided preliminary evidence of the effectiveness of vaccines in preventing severe cases of COVID-19 (i.e., reduced need for mechanical ventilation) at the national level.

International Scan

International guidance on post-vaccination activities was identified in 10 jurisdictions: Australia, Europe, England, Hong Kong, Israel, Netherlands, Norway, Singapore, UK, and US. 

  • Public Health Measures Post-Vaccination: In Australia, England, Hong Kong, and Singapore, public health measures remain in place regardless of vaccination status. In Europe, it is recommended that non-pharmaceutical interventions (NPIs) should be maintained irrespective of the vaccination status of individuals in public spaces and in large gatherings, including during travel.
  • Lifting Various Measures for the Vaccinated: Jurisdictions in Europe, Israel, Norway, and the US have lifted restrictions for those who are vaccinated in some circumstances. For example:
    • Europe: The ECDC suggests when fully vaccinated individuals meet other fully vaccinated individuals or when an unvaccinated individual(s) from the same household meet fully vaccinated individuals, physical distancing and the wearing of face masks can be relaxed if there are no risk factors for severe disease or lower VE in anyone present (e.g., older age, immunosuppression, other underlying conditions).  
    • Germany: As of May 9, 2021, those who are fully vaccinated as well as those who have recovered from COVID-19 infections can meet without restrictions, are not subject to nighttime curfews, and can visit stores, hairdressers, zoos, and other businesses without having to show a negative test result.  
    • US: Those who have been vaccinated are still expected to get tested if experiencing COVID-19 symptoms, and to follow Centers for Disease Control and Prevention (CDC) and health department travel requirements. HCWs who are fully vaccinated do not need to be restricted from work following higher-risk exposure.
    • Scotland: If the World Health Organization’s (WHO) six conditions for safe easing are met and all priority vaccination groups have been offered their first vaccine dose, restrictions will be decided on a regional rather than nationwide basis. Non-essential retail, pubs, restaurants, gyms, swimming pools, and other leisure activities can re-open.
  • Continued Contact Tracing for the Vaccinated when Exposed: The ECDC advises that vaccinated contacts who have been exposed to a confirmed case should continue to be managed according to existing ECDC guidance but health authorities may consider undertaking a risk assessment on a case-by-case basis (e.g., taking into consideration the local epidemiological situation in terms of circulating variants, the type of vaccine received, and the age of the contact).
  • Post-Vaccination Infection Surveillance: The Public Health England (PHE) Immunisation Department is conducting enhanced surveillance of cases of infection in vaccinated individuals in England. Clinicians who are seeing hospitalized patients face-to-face are also encouraged to report any confirmed cases in fully vaccinated individuals if they tested positive within the preceding seven days.
  • Cautions for Vaccinated Vulnerable Groups:
    • LTC Residents and Staff: Information about LTC restrictions was identified in the Netherlands, Hong Kong, and the US. For example, in the Netherlands, as of March 8, 2021, fully vaccinated residents of nursing homes can receive two visitors instead of one at the same time and these can be different persons over the week. In Hong Kong, vaccinated staff are exempt from routine testing.  
    • Post-Acute Care Settings: In the US, the CDC recommends that visitations in post-acute care settings should be permitted with some exceptions such as with unvaccinated residents, where the county COVID-19 positivity rate is >10% and <70% of residents in the facility are fully vaccinated. 
    • Immunosuppression or HIV: PHE advises that until further information becomes available vaccinated patients with immunosuppression should continue to follow advice to reduce the chance of exposure, and their adult household contacts should also be offered vaccination. In the US, the CDC recommends that health care facilities should continue to follow infection protection and control protocols for unvaccinated people, even when caring for vaccinated individuals with an immunocompromising condition. 
  • Travel-Related Recommendations: The ECDC suggests that requirements for testing and quarantine of travellers (if implemented) and regular testing at workplaces can be waived or modified for fully vaccinated individuals as long as there is no or very low level circulation of immune escape variants (in the community in the country of origin, in the case of travellers). Examples include: 
    • Australia: Mandatory quarantine with COVID-19 testing at frequent intervals is recommended for incoming international travelers and with interstate travel being governed by state/territory directives.
    • Hong Kong: Cross-boundary goods vehicle drivers who have completed a COVID-19 vaccination course are only required to possess proof of a negative test conducted within 72 hours. While travel between to/from Singapore is delayed, the organization of local group tours of not more than 30 persons including staff can resume. 
    • US: CDC prevention measures apply to all travelers, including those who are vaccinated. All travelers are required to wear a mask on all planes, buses, trains, and other forms of public transportation traveling into, within, or out of the US and including airports and stations.
  • Ensuring Equitable Access to Vaccines: In Europe, the ECDC advises that countries considering relaxing measures for fully vaccinated people should consider the potential for uneven inequitable vaccine access across the population.

Canadian Scan

Guidance on post-vaccination activities was identified in Canada, British Columbia (BC), Quebec, Northwest Territories (NWT), Prince Edward Island (PEI), Saskatchewan, and Yukon Territory.  

  • Recommended Activities Post-Vaccination: Whereas Canada’s National Advisory Committee on Immunization (May 3, 2021) advises that all public health measures should remain in place regardless of vaccination status, the Public Health Agency of Canada (May 14, 2021) advises that in the Summer of 2021, if 75% of eligible population has received first dose, and 20% have second dose, some public health measures may be relaxed while following local public health advice (i.e., physical distancing and mask wearing). Permitted activities might include small, outdoor gatherings, camping, hiking, and picnics, without crowds. In the Fall of 2021, if 75% of eligible population is fully vaccinated, local public health will be able to lift more measures including more activities indoors with people outside household bubbles.
    • In BC and PEI, everyone who receives the vaccine will still need to follow public health guidance and follow orders from the Provincial Health Officer, including washing hands, maintaining a safe physical distance, wearing a mask, and staying home when sick.
    • In Saskatchewan, provincial restrictions remain in place for all individuals with a three-phase reopening plan according to vaccination of age groups:  
      • Three weeks after 70% of individuals aged 40+ have received their first dose, many activities are permissible including: restaurants and bars may open, with a maximum of six people per table; 30% capacity or 150 people (whichever is less) at places of worship; group fitness classes with physical distancing; limit of 10 people at private gatherings (indoor/outdoor); limit of 30 people at public indoor gatherings; and a limit of 150 people at public outdoor gatherings.
      • Three weeks after 70% of individuals aged 30+ have received their first dose, permissible activities include: no capacity thresholds on retail/personal care services, but physical distancing must be maintained; 150-person limit at event facilities, casinos, bingo halls, theaters, libraries, and recreational facilities; 15 people at private indoor gatherings; and restrictions on adult/youth sports lifted. 
      • Three weeks after 70% of individuals aged 18+ have received their first dose, most remaining restrictions will be lifted and guidance on gathering sizes will be determined.
  • Recommendations for Activities After First Dose of Vaccine for LTC ResidentsThe province of Quebec found a 14-28-day interval necessary to reach optimal protection against the illness, depending on their age group (i.e., lower efficacy for long-term care residents during this period). Consequently, it is important to clearly inform vaccinated individuals about interval and individuals to avoid behaviours that increase their risk of infection after they are vaccinated.
  • Travel-Related Recommendations: NWT and Yukon introduced directives for travelers coming from within Canada. For example, as of April 21, 2021, people arriving in NWT from within Canada who are fully vaccinated can get tested on day eight of the mandatory 14-day self-isolation; if they test negative, they need only self-monitor for the remainder of the 14-day period. People who are fully vaccinated and returning from travel within Canada do not need to isolate from members of the same household who are also fully vaccinated. Members of the same household who are not fully vaccinated must self-isolate along with the returning traveler and may end the self-isolation period if the returning traveler tests negative after day eight.

Ontario Scan

  • No information was identified.


Individual peer-reviewed articles were identified through PubMed and Google Scholar. The search was limited to English sources and therefore may not capture the full extent of initiatives in non-English speaking countries. Full-text results extracted were limited to those available through Open Access or studies made available to the Ministry by our partners. Jurisdictional information was identified using Google and on relevant government websites. 

The Medical Subject Heading term “COVID-19” was used in combination with keywords to identify relevant articles for this review including: “activities”, “equity”, “infection”, “interactions”, “post-vaccination”, “public health measures”, and “risk”.  

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 developed this Evidence Synthesis Briefing Note: 

  • Evidence Synthesis Unit, Research Analysis and Evaluation Branch, Ministry of Health. May 28, 2021.