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#COVID-19 #Vaccines and #Vaccination

 Key Points

  • Available evidence suggests the currently authorized mRNA COVID-19 vaccines (Pfizer-BioNTech and Moderna) provide protection against a variety of strains, including B.1.1.7 (originally identified in the United Kingdom) and B.1.351 (originally identified in South Africa). Other vaccines show reduced efficacy against B.1.351 but may still protect against severe disease. Continued monitoring of vaccine effectiveness against variants is needed.
  • A growing body of evidence indicates that people fully vaccinated with an mRNA vaccine (Pfizer-BioNTech and Moderna) are less likely to have asymptomatic infection or to transmit SARS-CoV-2 to others. Studies are underway to learn more about the benefits of Johnson & Johnson/Janssen vaccine. However, the risk for SARS-CoV-2 infection in fully vaccinated people cannot be completely eliminated as long as there is continued community transmission of the virus.
  • At this time, there are limited data on vaccine effectiveness in people who are immunocompromised. People with immunocompromising conditions, including those taking immunosuppressive medications, should discuss the need for personal protective measures after vaccination with their healthcare provider.
  • This updated science brief synthesizes the scientific evidence supporting CDC’s guidance for fully vaccinated people and will continue to be updated as more information becomes available.


COVID-19 vaccination is a critical prevention measure to help end the COVID-19 pandemic. COVID-19 vaccines are now more widely accessible in the United States, and all people 12 years and older are recommended to be vaccinated against COVID-19. Three COVID-19 vaccines are currently authorized by the U.S. Food and Drug Administration (FDA) for emergency use: two mRNA vaccines (Pfizer-BioNTech, Moderna) and one viral vector vaccine (Johnson & Johnson/Janssen vaccine). People are considered fully vaccinated if they are ≥2 weeks following receipt of the second dose in a 2-dose series (mRNA vaccines), or ≥2 weeks following receipt of a single-dose vaccine (Johnson & Johnson/Janssen).*

Public health recommendations for people fully vaccinated with COVID-19 vaccines must consider the evidence, including vaccine effectiveness against symptomatic and asymptomatic COVID-19, as well as vaccine impact on SARS-CoV-2 transmission. Other individual and societal factors are also important when evaluating the benefits and potential harms of prevention measures among vaccinated individuals. The Advisory Committee on Immunization Practices and CDC routinely consider factors such as population values, acceptability, and feasibility of implementation when making vaccine recommendations.(1) These factors were also considered when developing CDC’s interim public health recommendations for fully vaccinated people.

In this scientific brief, we summarize evidence available through May 19, 2021, for the currently authorized COVID-19 vaccines (administered according to the recommended schedules) and additional considerations used to inform public health recommendations for fully vaccinated people, including:

  • Vaccine efficacy and effectiveness against SARS-CoV-2 infection
  • Vaccine performance against emerging SARS-CoV-2 variant viruses
  • Impact of other prevention measures in the context of vaccination

Accumulating evidence indicates that fully vaccinated people without immunocompromising conditions are able to engage in most activities with very low risk of acquiring or transmitting SARS-CoV-2. The benefits of avoiding disruptions such as unnecessary quarantine and social isolation might outweigh the low residual risk of becoming ill with COVID-19, generally with mild disease, or of transmitting the virus to others.


COVID-19 vaccine efficacy and effectiveness

Vaccine efficacy refers to how well a vaccine performs in a carefully controlled clinical trial, whereas effectiveness describes its performance in real-world observational studies. Evidence demonstrates that the authorized COVID-19 vaccines are both efficacious and effective against symptomatic, laboratory-confirmed COVID-19, including severe forms of the disease. In addition, a growing body of evidence suggests that mRNA COVID-19 vaccines also reduce asymptomatic infection and transmission. Substantial reductions in SARS-CoV-2 infections (both symptomatic and asymptomatic) will reduce overall levels of disease, and therefore, viral transmission in the United States. However, investigations are ongoing to assess further the impact of COVID-19 vaccination on transmission.

Animal challenge studies

Rhesus macaque challenge studies provided the first evidence of the potential protective effects of Pfizer-BioNTech, Moderna, and Johnson & Johnson/Janssen COVID-19 vaccines against SARS-CoV-2 infection, including asymptomatic infection. Vaccinated macaques developed neutralizing antibodies that exceeded those in human convalescent sera and showed no or minimal signs of clinical disease after SARS-CoV-2 challenge.(2-4) In addition, COVID-19 vaccination prevented or limited viral replication in the upper and lower respiratory tracts, which may have implications for transmission of the virus among humans.(2-4)

Vaccine efficacy from human clinical trials

Clinical trials subsequently demonstrated the authorized COVID-19 vaccines to be efficacious against laboratory-confirmed, symptomatic COVID-19 in adults, including severe forms of the disease, with evidence for protection against asymptomatic SARS-CoV-2 infection as well (5-11) (Box). Recent trial data demonstrated 100% efficacy of the Pfizer-BioNTech vaccine against laboratory-confirmed, symptomatic COVID-19 in adolescents 12–15 years old, although this estimate was based on small numbers of cases.(12)

Box 1. Summary of vaccine efficacy estimates for authorized COVID-19 vaccines

All authorized COVID-19 vaccines demonstrated efficacy (range 65% to 95%) against symptomatic, laboratory-confirmed COVID-19 in adults ≥18 years.

  • For each authorized COVID-19 vaccine, the overall efficacy was similar to the efficacy across different populations, including elderly and younger adults, in people with and without underlying health conditions, and in people representing different races and ethnicities.
  • The Pfizer-BioNTech COVID-19 vaccine also demonstrated high efficacy against symptomatic, laboratory-confirmed COVID-19 in adolescents aged 12-17 years.

All authorized COVID-19 vaccines demonstrated high efficacy (≥89%) against COVID-19 severe enough to require hospitalization.

All authorized COVID-19 vaccines demonstrated high efficacy against COVID-19-associated death.

  • In the clinical trials, no participants who received a COVID-19 vaccine died from COVID-19; the Moderna and Johnson & Johnson/Janssen trials among adults ≥18 years each had COVID-19 deaths in the placebo arm.

Preliminary data from the clinical trials among adults ≥18 years old suggest COVID-19 vaccination may also protect against asymptomatic infection.

  • In the Moderna trial, among people who had received a first dose, the number of asymptomatic people who tested positive for SARS-CoV-2 at their second-dose appointment was approximately two-thirds lower among vaccinees than among placebo recipients (0.1% and 0.3%, respectively)
  • Efficacy of Johnson & Johnson/Janssen COVID-19 vaccine against asymptomatic seroconversion was 74% in a subset of trial participants.

No trials have compared efficacy between any of the authorized vaccines in the same study population at the same time, making comparisons of efficacy difficult.

  • All Phase 3 trials differed by calendar time and geography.

Real-world vaccine effectiveness

Multiple studies from the United States and other countries demonstrate that a two-dose COVID-19 vaccination series is highly effective against SARS-CoV-2 infection (including both symptomatic and asymptomatic infections) and sequelae including severe disease, hospitalization, and death. Early evidence for the Johnson & Johnson/Janssen vaccine also demonstrates effectiveness against COVID-19 in real-world conditions.

Table 1a. Effectiveness against SARS-CoV-2 infection and symptomatic disease

CountryPopulationVaccineOutcomeVaccine Effectiveness
United States13General adult populationPfizer-BioNTech or ModernaSARS-CoV-2 infection89%*1
United States14General adult populationPfizer-BioNTech or ModernaSARS-CoV-2 infection86%*2
United States15General adult populationPfizer-BioNTech or ModernaHospitalization96%*1
United States16Healthcare workersPfizer-BioNTechSARS-CoV-2 infection97%*2
ModernaSARS-CoV-2 infection99%*2
United States17Healthcare workers, first responders, and other essential and frontline workersPfizer-BioNTech or ModernaSARS-CoV-2 infection90%*2
United States18Healthcare WorkersPfizer-BioNTechSARS-CoV-2 infection96%*1
United States(19)Healthcare workersPfizer-BioNTech or ModernaSymptomatic disease94%*1
United States(20)Healthcare workers and residents in a skilled nursing facilityPfizer-BioNTechResidents: symptomatic disease87%*2
Residents: hospitalization94%*2
Healthcare workers: symptomatic disease87%*2
United States21Hospitalized adults ≥65 years oldPfizer-BioNTech or ModernaHospitalization94%*2
United States22Health system members ≥18 years oldJohnson & Johnson/JanssenSARS-CoV-2 infection77%*2
United Kingdom23Healthcare workersPfizer-BioNTech or AstraZenecaSARS-CoV-2 infection90%*2
United Kingdom24Healthcare workersPfizer-BioNTechSARS-CoV-2 infection86%*1
United Kingdom (Scotland)26Healthcare workersPfizer-BioNTech or AstraZenecaSARS-CoV-2 infection92%*2
United Kingdom27Adults aged ≥ 80 years, including those with multiple underlying conditionsPfizer-BioNTechSymptomatic disease85%*2
Israel28HMO members >16 years oldPfizer-BioNTechSARS-CoV-2 infection89%*1
Israel29Health system membersPfizer-BioNTech<60 years old: SARS-CoV-2 infection93%*2
≥60 years old: SARS-CoV-2 infection92%*2
Israel30General adult populationPfizer-BioNTechSARS-CoV-2 infection92%*1
Symptomatic disease94%*1
Severe disease92%*1
Israel31General population ≥16 yearsPfizer-BioNTechSARS-CoV-2 infection93%*1
Severe disease94%*1
Israel32General population ≥16 yearsPfizer-BioNTechSymptomatic disease>97%*1
Severe/critical disease98%*
Israel33Healthcare workersPfizer-BioNTechSymptomatic disease>97%*1
Israel34Healthcare workersPfizer-BioNTechSymptomatic disease>90%*3
Italy35Healthcare workersPfizer-BioNTechSymptomatic disease>95%*1
Denmark36Long term care facility residentsPfizer-BioNTechSARS-CoV-2 infection64%*1
Long term care facility staffPfizer-BioNTechSARS-CoV-2 infection90%*1
Sweden37General adult populationPfizer-BioNTechSARS-CoV-2 infection86%*1

*Only studies including estimates of vaccine effectiveness ≥7 days following a completed vaccination series are included here. Studies examining multiple vaccines for which a single estimate of vaccine effectiveness is reported did not assess vaccine effectiveness by product type.

1≥7 days after second dose

2≥14 days after second dose

3≥11 days after second dose

In addition to the studies listed in Table 1a., further evidence of the impact of vaccination with Pfizer-BioNTech and Moderna COVID-19 vaccine has been demonstrated among healthcare workers, with major reductions in SARS-CoV-2 infections among those receiving two doses of COVID-19 vaccine even when community transmission was increasing.(38-40)

Table 1b. Effectiveness of COVID-19 Vaccination Against Asymptomatic SARS-CoV-2 Infection

CountryPopulationVaccineOutcomeVaccine effectiveness
United States49General adult populationPfizer-BioNTech or ModernaAsymptomatic infection80%*1
United States18Healthcare workersPfizer-BioNTechAsymptomatic infection90%*2
Israel32General adult populationPfizer-BioNTechAsymptomatic infection92%*2
Israel33Healthcare workersPfizer-BioNTech or AstraZenecaAsymptomatic infection86%*2
Israel34Healthcare workersPfizer-BioNTechAsymptomatic infection65%*3

1≥0 days after second dose
2≥7 days after second dose
3≥11 days after second dose

Data from multiple studies in different countries suggest that people vaccinated with Pfizer-BioNTech COVID-19 vaccine who develop COVID-19 have a lower viral load than unvaccinated people.(50-54) This observation may indicate reduced transmissibility, as viral load has been identified as a key driver of transmission(55). Two studies from the United Kingdom found significantly reduced likelihood of transmission to household contacts from people infected with SARS-CoV-2 who were previously vaccinated for COVID-19.(26, 56)

Vaccine effectiveness in immunosuppressed people

Evidence of reduced antibody response to or reduced immunogenicity of COVID-19 mRNA vaccination has been observed in the following groups: people taking certain immunosuppressive medications like rituximab (42, 43, 45, 57) or mycophenolate (45, 58-60), people with hematologic cancers (44, 47), and hemodialysis patients (46). At this time, data on vaccine protection in people who are immunocompromised are limited; in addition, the impact of immune suppression on COVID-19 vaccine effectiveness may vary by condition.(47, 48) Complete data on which immunocompromising conditions might affect response to COVID-19 vaccination are not available; in addition, there is no established immune correlate of protection against SARS-CoV-2 so the risk of infection in people who respond incompletely to COVID-19 vaccination cannot be quantified using immunogenicity data. People with immunocompromising conditions, including those taking immunosuppressive medications, should discuss the need for personal protective measures after vaccination with their healthcare provider.

Vaccine performance against emerging SARS-CoV-2 variant viruses

SARS-CoV-2 variants of concern (VOC: B.1.1.7, first detected in the United Kingdom; B.1.351, first detected in South Africa; P.1, first detected in Japan/Brazil; and B.1.427 and B.1.429, first detected in US-California) have emerged with mutations that alter the receptor binding domain of the spike protein (notably the N501Y mutation occurring in B.1.1.7, B.1.351 and P.1 variants, the E484K and E417T/N mutations in B.1.351 and P.1, and the L452R mutation in B.1.427 and B.1.429).(61) Similar mutations also occur in SARS-CoV-2 variants of interest (VOI: B.1.526 and B.1.526.1, first detected in US-New York; B.1.525, first detected in the United Kingdom/Nigeria; and B.1.617, B.1.617.1, B.1.617.2 and B.1.617.3, first detected in India)(61), but these variants currently have limited prevalence or expansion in the United States or other countries and still lack clear evidence of increased transmission, disease severity, or impact on available vaccines, therapeutics, or diagnostic tests.(61) Vaccine performance against emerging SARS-CoV-2 variants is an important consideration when evaluating the need for prevention measures in vaccinated people and will require continued monitoring. When evaluating risk, considering regional and local circulation of SARS-CoV-2 variants is also relevant; current data can be found on CDC’s website.

Vaccine-induced neutralizing antibody activity

Sera from mRNA COVID-19 vaccine (both Pfizer-BioNTech and Moderna) recipients have demonstrated minimal to large reductions in antibody neutralization activity against a variety of mutations.(62-106) Across studies of VOC, the greatest reductions were observed for B.1.351, followed by P.1; reductions for B.1.1.7 and B.1.427/B.1.429 were minimal. A limited number of studies were available for some VOI that demonstrated greater reductions for P.2, B.1.525, and B.1.617.1 and minimal reductions for B.1.617 and B.1.526.(67, 84, 86-88, 93-95, 100, 101) The E484K mutation alone or in combination with other mutations in the receptor binding domain has been shown to account for the majority of reduction in vaccine-induced neutralizing antibody activity for the B.1.351, P.1, and P.2 variants.(65, 67, 77, 79, 107) B.1.1.7 and B.1.526 variants with E484K mutations, which have been detected in the United Kingdom, United States, and other countries, have shown further reductions in neutralization above B.1.1.7 and B.1.526 alone, respectively.(62, 65, 81, 84, 86, 93) Two studies have shown that six months after receiving the Moderna vaccine, higher proportions of people had undetectable neutralization activity against B.1.351 and P.1 compared with the ancestral strain.(108, 109) In the absence of a biological correlate of protection, it is difficult to predict how reduced neutralizing activity may affect COVID-19 vaccine effectiveness. However, across studies, antibody neutralizing activity of sera from vaccinated people was generally higher than that observed for convalescent sera from people who have recovered from COVID-19.(65, 66, 68, 72-77, 79, 83, 104, 107)

Vaccine-induced cellular immunity

Several studies have assessed CD4+ and CD8+ T cell responses from Moderna or Pfizer vaccine recipients to the ancestral SARS-CoV-2 strain compared with the B.1.1.7, B.1.351, P.1, and B.1.427 variants; these studies observed modest or no defects in cellular immune recognition of the variants.(75, 90, 101, 110-113) Thus, cellular immunity may help limit disease severity in infections caused by variants that partially escape neutralizing antibodies. Polymorphisms in human leukocyte antigen alleles have been observed to result in variation of the T cell response to specific variants, which may impact different subpopulations differently based on higher genetic prevalence.(114-116)

Efficacy and effectiveness

A recent study from Qatar demonstrated high effectiveness after ≥14 days for the Pfizer-BioNTech vaccine against any documented infection caused by B.1.1.7 (90%) and B.1.351 (75%); importantly, the vaccine was 100% effective against severe, critical, or fatal disease, regardless of strain.(117) As described above, a growing number of studies in Israel, Europe, and the United Kingdom have also demonstrated high real-world effectiveness (>85%) of two doses of Pfizer-BioNTech COVID-19 vaccine while B.1.1.7 was prevalent.(24, 30-32, 34, 37)  A study from California demonstrated 86% effectiveness among people fully vaccinated with Pfizer-BioNTech or Moderna vaccine during a time when 69% of sequenced SARS-CoV-2 isolates in the state were B.1.1.7, B.1.427, or B.1.429.(14)  Clinical trial data suggest that the Johnson & Johnson/Janssen COVID-19 vaccine may have reduced overall efficacy against the B.1.351 variant.  Although spike-protein-specific and seroresponse rates were similar between U.S clinical trial participants and those from Brazil and South Africa, vaccine efficacy after ≥14 days was 74% in the United States and 66% in Brazil (where ~69% of infections were due to P.2), but in South Africa (~where 95% of infections were due to B.1.351), efficacy was 52%. (8, 9) (11) Notably, Johnson & Johnson/Janssen vaccine showed good efficacy against severe or critical disease (73%–82%) across all sites. (8, 9)

Impact of prevention measures in the context of vaccination

Individual and community-level prevention measures have been shown to help reduce the spread of SARS-CoV-2. (118-123) However, there are individual and societal costs related to physical distancing, quarantine, school and business closures, and other prevention measures.(124-131)

Modeling studies suggest that adherence to prevention measures, such as wearing masks and physical distancing, continues to be important in the context of vaccine implementation.(132-140) In one study, complete relaxation of prevention measures for the entire population prior to adequate vaccination coverage (60-80% depending on the population considered) resulted in essentially no reductions in SARS-CoV-2 infections.(132) However, in the context of rapid vaccine implementation, the benefit of non-pharmaceutical interventions decreases: preliminary data from one study found prevention measures in the United States could begin to be relaxed 2-3 months after vaccination began if a rate of 3 million doses administered daily were attained.(141) Correspondingly, preliminary data suggest that increasing vaccination rates may allow for the phasing out of some prevention measures as coverage increases (138). With high vaccine effectiveness and increasing vaccination coverage, preliminary modeling studies predict that vaccinated people returning to normal activities will have minimal impact on the course of the pandemic.(141, 142)

In summary, prevention measures will continue to be important during the period of vaccine deployment. As vaccination coverage increases, phasing out prevention measures for fully vaccinated people, ideally those measures that are the most disruptive to individuals and society, will be increasingly feasible.


COVID-19 vaccines currently authorized in the United States have been shown to be efficacious and effective against SARS-CoV-2 infections, including asymptomatic infection, symptomatic disease, severe disease, and death. These findings, along with the early evidence for reduced viral load in vaccinated people who develop COVID-19, suggest that any associated transmission risk is likely to be substantially reduced in vaccinated people. While vaccine effectiveness against emerging SARS-CoV-2 variants remains under investigation, available evidence suggests that the COVID-19 vaccines presently authorized in the United States offer protection against known emerging variants.

Evidence suggests the U.S. COVID-19 vaccination program has the potential to substantially reduce the burden of disease in the United States by preventing illness in fully vaccinated people and interrupting chains of transmission. The risks of SARS-CoV-2 infection in fully vaccinated people cannot be completely eliminated where community transmission of the virus is widespread. Vaccinated people could potentially still become infected and spread the virus to others. However, in the context of rapidly increasing vaccination coverage, modeling data predict reduced benefits of non-pharmaceutical prevention measures and minimal impact on the course of the pandemic of fully vaccinated people returning to normal activities. People with compromised immune systems should discuss the need to continue using personal protective measures after vaccination with their healthcare provider. Some fully vaccinated people who are not immunocompromised may prefer to continue using prevention measures for personal comfort or because they or a member of their family are at increased risk for severe COVID-19. Taken together, the evidence supports phasing out prevention measures for fully vaccinated people as an increasingly large proportion of the United States population receives COVID-19 vaccines.

*Note: CDC guidance for fully vaccinated people can also be applied to COVID-19 vaccines that have been listed for emergency use by the World Health Organization (e.g. AstraZeneca/Oxford). This brief summarizes evidence related to vaccines authorized for emergency use in the United States.


Note: Preprints have not been peer-reviewed. They should not be regarded as conclusive, guide clinical practice/health-related behavior, or be reported in news media as established information.

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