What We Know About COVID-19 Vaccine Effectiveness Against Variants

Kirsten Yates, MPH; Ruth Link-Gelles, PhD, MPH; Heather Scobie, PhD, MPH; Janine Cory, MPH


May 03, 2022

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What do available data tell us about COVID-19 vaccine effectiveness against infection, hospitalization, and death, including during the emergence of new variants like Omicron?

All available data show that mRNA COVID-19 vaccines continue to meet their primary goal: prevention of severe outcomes from COVID-19, including hospitalization and death, among children and adults. Protection against severe disease is highest in those who receive a booster dose.[1,2,3]

Although the level of protection against SARS-CoV-2 infection after COVID-19 vaccination has declined in all ages with the emergence of the Omicron variant, vaccination still reduces the risk for infection and COVID-19–associated hospitalization and death, especially among people who receive a booster dose.

What are the sources of data that CDC has about COVID-19 vaccine effectiveness?

CDC uses a multilayered approach to understand how well COVID-19 vaccines work in the real world. This approach includes ongoing analyses of surveillance data and studies of vaccine effectiveness. Surveillance data allow us to monitor rates of COVID-19 cases, hospitalizations, and deaths by vaccination status. This information can signal potential changes in vaccine effectiveness that can be further studied through more robust, controlled vaccine effectiveness studies conducted by CDC or our partners. These studies provide vaccine effectiveness estimates that help us better understand how well the vaccines work in different groups of people, against different outcomes, and different variants.

Surveillance Data

CDC uses the following platforms and methods to collect and analyze surveillance data on COVID-19 cases, hospitalizations, and deaths by vaccination status:

  • Case surveillance: CDC collaborates with 30 public health jurisdictions that actively link immunization registry and case surveillance data to monitor rates of COVID-19 cases and deaths by vaccination status, including receipt of a booster dose.

  • COVID-NET: CDC tracks COVID-19 hospitalizations by vaccination status using COVID-NET. COVID-NET is a population-based, sentinel surveillance system that collects reports of laboratory-confirmed COVID-19–associated hospitalizations in 99 counties in 14 states.

  • National Healthcare Safety Network: Infections in long-term care facility residents by vaccination status are reported through a network of approximately 15,000 medical facilities called the National Healthcare Safety Network (NHSN).

Vaccine Effectiveness Studies

Vaccine effectiveness studies build on surveillance data by collecting extra information on cases (eg, individuals with COVID-19) and controls (eg, individuals without COVID-19) in more defined settings, such as specific hospitals, allowing for comparisons of these groups. Information such as a history of previous SARS-CoV-2 infection and underlying conditions (eg, cancer, diabetes) collected as part of these studies helps CDC gain more precise estimates and a better understanding of how well vaccines are working. Vaccines were initially prioritized for healthcare providers who were at higher risk for exposure to SARS-CoV-2, as well as older adults. Vaccine effectiveness studies allow us to compare differences among people who are vaccinated and those who are not vaccinated to ensure we account for how differences such as age or risk for exposure affect, or bias, estimates of vaccine effectiveness. This is why we often present vaccine effectiveness data by age group or by specific populations, such as healthcare personnel.

CDC works with partners to study COVID-19 vaccine effectiveness using several data collection platforms and study designs. Vaccine effectiveness studies vary on the basis of outcome (eg, infection, hospitalization), population (eg, focusing on residents of long-term care facilities), and study design. For example, CDC partners with the VISION Network, which includes 306 emergency department and urgent care clinics and 164 hospitals in 10 states, to study how well vaccines protect against COVID-19–associated emergency department/urgent care visits and hospitalizations in children and adults. Another study platform, Overcoming COVID, monitors vaccine effectiveness in children aged 18 years or younger who are hospitalized with symptoms of COVID-19. Results from these ongoing studies are regularly published in CDC's MMWR , as well as other scientific journals.[4] The Overcoming COVID platform can also evaluate vaccine effectiveness in children younger than 5 years, should COVID-19 vaccines become available for this younger age group.

For more information on the current and planned vaccine effectiveness studies that CDC is currently conducting with partners, visit COVID-19 Vaccine Effectiveness Research.

How comprehensive are the data, and what types of data may be missing? Do we have sufficient data about how vaccinated persons in the US did during different waves of infections (pre-Delta vs Delta vs Omicron)?

CDC uses multiple important sources for data. No single study or surveillance system can provide all the information needed to understand how well COVID-19 vaccines work in real-world conditions over time, which is why this multilayered approach is so important.

CDC's multijurisdictional surveillance data, which tracks rates of COVID-19 cases and associated deaths by vaccination status, is extensive, covering more than 65% of the US population. The volume of this data allows for analysis of outcomes over time, such as COVID-19–associated deaths. However, although these data signal potential changes in vaccine effectiveness, they cannot control for all factors that may affect the rates being tracked — for example, prior infection.

Conversely, the smaller study populations included in vaccine effectiveness studies allow us to control for some of these variables. However, sometimes the small number of outcomes observed within certain age and vaccination categories reduce the precision of vaccine effectiveness estimates. Although this isn't ideal from a data analysis standpoint, it can sometimes be a good thing from a public health perspective. For example, when there are so few hospitalizations among vaccinated children in a study population that researchers can't calculate statistically significant vaccine effectiveness estimates, we can take that as an indication that these vaccines are likely succeeding at what they were designed to do — keeping children from getting sick enough to need hospitalization.

CDC's multilayered approach to monitoring has also allowed us to quickly identify how different variants affect vaccine effectiveness, including during the Omicron wave. Researchers can stratify analyses by variant period, allowing CDC to compare vaccine effectiveness estimates or rates of cases and hospitalizations by vaccination status during different variant periods. Some vaccine effectiveness studies have also used whole-genome sequencing to confirm which variants are associated with the different COVID-19 outcomes being studied.

Is there significant variation in vaccine effectiveness based on the type of vaccine product received?

Studies show that, in general, mRNA COVID-19 vaccines (Pfizer-BioNTech and Moderna) provide higher levels of protection against all outcomes than the J&J/Janssen COVID-19 vaccine.[5] Although analyses of jurisdictional surveillance data may appear to suggest that protection has recently increased among people who received the Janssen vaccine to similar levels as in those who received an mRNA vaccine, many Janssen vaccine recipients have received a booster dose of an mRNA vaccine or have received additional protection through infection with COVID-19 — which could potentially bias surveillance results.

A separate vaccine effectiveness study[5] found that people who received the J&J/Janssen COVID-19 vaccine as both their first dose and booster dose had lower levels of protection against COVID-19–associated emergency department and urgent care visits during Omicron compared with those who received an mRNA booster dose. In addition, people who received an mRNA primary series and booster had the most protection against COVID-19–associated hospitalization.

CDC recommends mRNA COVID-19 vaccines over the Janssen vaccine for primary and booster vaccination for all eligible people in most situations. However, offering the Janssen COVID-19 vaccine is preferable to not providing any COVID-19 vaccine.

What should healthcare providers tell their patients about how well COVID-19 vaccines are working?

At this point, most of your patients have either heard about or experienced SARS-CoV-2 "breakthrough" infections with the highly contagious Omicron variant, which can understandably weaken confidence in the effectiveness of vaccination. Helping them understand the difference between preventing infection and preventing severe disease — which can cause hospitalization and death — is a good way to start to frame the conversation when people question vaccine effectiveness.

Although people frequently consult family members, friends, and webpages for information on vaccines, doctors are consistently ranked as the most trusted source for vaccine information. With this unique position, your strong recommendation is critical for patient education and vaccine acceptance.

Here are a few talking points to help build COVID-19 vaccine confidence when talking with your patients:

  • COVID-19 vaccines are continuing to meet their primary goal: keeping people out of emergency rooms, hospitals, and intensive care units, as well as preventing death. Although people who are vaccinated may still get infected, they are much less likely to get severely ill compared with unvaccinated people.[6]

  • Make sure you and your eligible family members stay up-to-date on COVID-19 vaccines. Staying up-to-date protects you and your eligible children from severe outcomes related to COVID-19, including hospitalization and death.

  • It also helps your family and your community directly. Hospitals need to have the ability to treat and manage all kinds of diseases. If they are overwhelmed with patients with COVID-19, it can jeopardize healthcare for anyone who needs intensive care or has a medical emergency like a heart attack.

  • Boosters work. We know that, for adults, booster doses do exactly what it sounds like — "boost" the immune system's protection against serious illness. We don't have enough data yet to know whether kids aged 5-11 years will need boosters, but as your healthcare provider, I encourage you and your family to stay up-to-date on your COVID-19 vaccines.

  • Vaccine and masking recommendations can change when we get new data, especially with a new disease like COVID-19 and when new variants arise. When CDC and scientists learn that people might need booster doses, or new variants emerge, they look very carefully at the data to make or change vaccine recommendations. I feel strongly that staying up-to-date on your COVID-19 vaccines, including getting boosters when they are recommended, is one of the best ways to protect you and your family from severe COVID-19.

CDC Resources

CDC COVID Data Tracker: Rates of COVID-19 Cases and Deaths by Vaccination Status 

CDC COVID Data Tracker: Rates of Laboratory-Confirmed COVID-19 Hospitalizations by Vaccination Status  

CDC COVID Data Tracker: COVID-19 Vaccine Effectiveness

COVID-19 Vaccine Breakthrough Case Investigation and Reporting

The Possibility of COVID-19 after Vaccination: Breakthrough Infections 

Benefits of Getting a COVID-19 Vaccine

COVID-19 Vaccines Work


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