Why are boosted Americans testing positive for COVID more than those without extra shot?

Authors: Julia Marnin, McClatchy Washington Bureau June 7, 2022

Since late February, Americans who have gotten a booster shot appear to be testing positive for COVID-19 more often than those vaccinated without the extra shot, according to Centers for Disease Control and Prevention data.

This is based on numbers up until the week of April 23, which is the most recently released CDC data comparing case rates of those boosted, vaccinated and unvaccinated against the coronavirus. Ultimately, the numbers, which are updated monthly, showed those unvaccinated had the highest case rates overall.

Meanwhile, about 119 out of 100,000 boosted individuals tested positive for COVID-19 during the week of April 23, according to CDC data. In comparison, 56 out of 100,000 individuals vaccinated with only a primary series tested positive.

But why are the case rates higher for boosted individuals than for those vaccinated without a booster?

Dr. Sheela Shenoi, an infectious disease doctor and assistant professor at Yale School of Medicine, told McClatchy News over the phone that “there’s no biological reason that people who have had (the vaccine) and boosters are going to be at increased risk for COVID.”

“These numbers are not telling us the whole truth,” Shenoi said.

The CDC wrote in a summary accompanying its data that “several factors likely affect crude case rates” and this makes “interpretation of recent trends difficult.”

Here are some potential factors to keep in mind, according to health experts, when looking at the data.

At-home testing

“The wide availability of at-home tests has substantially muddied the waters, because these do not necessarily show up in official figures,” Bill Hanage, an associate professor of epidemiology at Harvard University’s T.H. Chan School of Public Health and a co-director for the Center for Communicable Disease Dynamics, told McClatchy News in a statement.

“Individuals receiving boosters may be more likely to have their cases counted,” Hanage said.

Hanage said this is because “just in being boosted, they are displaying ‘health seeking’ behavior” and “they are more likely to have contact with healthcare and get a test that ends up in official stats.”

In the U.S., more than 221 million people are fully vaccinated and more than 103 million of those people have received their first booster dose as of June 7, according to the CDC.

Those vaccinated without a booster “are more likely young, and so less likely to be severely ill in general,” Hanage said. “If they do a rapid test, they may not report it. They may not even do a test.”

Dr. Wafaa El-Sadr, a professor of epidemiology and medicine at Columbia University’s Mailman School of Public Health and director of ICAP, told McClatchy News over the phone that the wide availability of self testing “has completely changed the picture overall.”

“We don’t know the number of tests that are done and we don’t know how many are positive, how many are negative,” El-Sadr said. “So it’s a whole kind of black box that makes looking at case rates really very unreliable.”


Shenoi said it’s possible that individual behaviors might influence why CDC data shows those with booster shots are testing positive more than those vaccinated with a primary vaccine series.

Those boosted may feel more comfortable and safe, according to Shenoi, and as a result, they might be taking less COVID-19 precautions such as masking and social distancing “because they feel like they’re protected by the booster.”

Over the past few months, Shenoi said the country has seen “people getting infections, although overwhelmingly mild, thankfully, and that may correlate with people feeling that they’re protected and engaging in kind of their normal activities where they may be more exposed to other people with COVID and facilitating spread.”

El-Sadr said the issue with the CDC case rate data is that it depends “very much on behaviors, whether it be testing behavior” or “the characteristics of people who are boosted versus people who are not boosted.”

Prior infections and those at higher risk

It’s possible that people who have gotten their primary vaccine series but not a booster “are more likely to have been recently infected during the first omicron wave,” Hanage said.

With that “additional immunity from that infection,” they are less likely to be infected now, he added.

Dr. Peter Gulick, an associate professor of medicine at Michigan State University and the director of its Internal Medicine Osteopathic Residency Program, told McClatchy News in a statement that “one thing to consider is the group that gets the boosters.”

He described this group as older, immunocompromised individuals who are at a higher risk when it comes to COVID-19 breakthrough infections because of their “only partial response (to the) vaccines.”

El-Sadr also said those who are boosted may be at a higher risk of testing positive for COVID-19.

Because of this, “you can’t really attribute their higher risk of getting infected with COVID to the booster.”

The CDC’s late April case rate data was recorded when COVID-19 cases were trending upward in the U.S. due to the omicron variant and its subvariants.

The omicron variant, which is highly infectious and generally causes less severe symptoms compared with other variants, continues to dominate cases in the U.S. as of June 4, according to the CDC.

Shenoi said she predicts cases are going to continue to rise in the summer based on how infectious the variant and its subvariants are and how Americans appear ready to “move on and get back to their normal lives.”

Did you test positive for COVID at home? Here’s what to do next

Coronavirus weekly need-to-know: COVID infection risks, Paxlovid, cases, variants & more

COVID infection risk lower for those with food allergies, study ‘unexpectedly’ finds

1 in 5 adults may develop long COVID symptoms after infection, CDC says

Potential for False Positive Results with Antigen Tests for Rapid Detection of SARS-CoV-2 – Letter to Clinical Laboratory Staff and Health Care Providers

Authors: FDA

The U.S. Food and Drug Administration (FDA) is alerting clinical laboratory staff and health care providers that false positive results can occur with antigen tests, including when users do not follow the instructions for use of antigen tests for the rapid detection of SARS-CoV-2. Generally, antigen tests are indicated for the qualitative detection of SARS-CoV-2 antigens in authorized specimen types collected from individuals who are suspected of COVID-19 by their healthcare provider within a certain number of days of symptom onset. The FDA is aware of reports of false positive results associated with antigen tests used in nursing homes and other settings and continues to monitor and evaluate these reports and other available information about device safety and performance.

The FDA reminds clinical laboratory staff and health care providers about the risk of false positive results with all laboratory tests. Laboratories should expect some false positive results to occur even when very accurate tests are used for screening large populations with a low prevalence of infection. Health care providers and clinical laboratory staff can help ensure accurate reporting of test results by following the authorized instructions for use of a test and key steps in the testing process as recommended by the Centers for Disease Control and Prevention (CDC), including routine follow-up testing (reflex testing) with a molecular assay when appropriate, and by considering the expected occurrence of false positive results when interpreting test results in their patient populations.  

For More Information: https://www.fda.gov/medical-devices/letters-health-care-providers/potential-false-positive-results-antigen-tests-rapid-detection-sars-cov-2-letter-clinical-laboratory

Antigen and Molecular Tests

Diagnostic tests are used to detect current, active infections of the SARS-CoV-2 virus. Diagnostic tests can be antigen based (“rapid antigen” tests), which look for protein markers on the outside of the virus, or they can be molecular based (including PCR, LAMP, CRISPR), which look for viral genomic material specific to SARS-CoV-2. Molecular based tests that amplify genetic material are also called nucleic acid amplification tests (NAAT). 

More about antigen and molecular tests:

For More Information: https://www.centerforhealthsecurity.org/covid-19TestingToolkit/testing-basics/types-of-COVID-19-tests/antigen-and-molecular-tests.html

Performance of an Antigen-Based Test for Asymptomatic and Symptomatic SARS-CoV-2 Testing at Two University Campuses

Authors: Ian W. Pray, PhD1,2,3,*; Laura Ford, PhD1,2,*; Devlin Cole, MD3,4; Christine Lee, PhD1,5; John Paul Bigouette, PhD1,2; Glen R. Abedi, MPH1; Dena Bushman, MSN, MPH1,2; Miranda J. Delahoy, PhD1,2; Dustin Currie, PhD1,2; Blake Cherney, MS1; Marie Kirby, PhD1; Geroncio Fajardo, MD1; Motria Caudill, PhD1,6; Kimberly Langolf, MS7; Juliana Kahrs, MS7; Patrick Kelly, MD4,8; Collin Pitts, MD4,8; Ailam Lim, PhD9; Nicole Aulik, PhD9; Azaibi Tamin, PhD1; Jennifer L. Harcourt, PhD1; Krista Queen, PhD1; Jing Zhang, PhD1; Brett Whitaker, PhD1; Hannah Browne1; Magdalena Medrzycki, PhD1; Patricia Shewmaker, PhD1; Jennifer Folster, PhD1; Bettina Bankamp, PhD1; Michael D. Bowen, PhD1; Natalie J. Thornburg, PhD1; Kimberly Goffard, MBA10; Brandi Limbago, PhD1; Allen Bateman, PhD7,11; Jacqueline E. Tate, PhD1; Douglas Gieryn10; Hannah L. Kirking, MD1; Ryan Westergaard, MD, PhD3,4; Marie Killerby, VetMB1; CDC COVID-19 Surge Laboratory Group (View author affiliations)


What is already known about this topic?

Antigen tests for SARS-CoV-2 are inexpensive and can return results within 15 minutes, but test performance data in asymptomatic and symptomatic persons are limited.

What is added by this report?

Compared with real-time reverse transcription–polymerase chain reaction (RT-PCR) testing, the Sofia antigen test had a sensitivity of 80.0% and specificity of 98.9% among symptomatic persons; accuracy was lower (sensitivity 41.2% and specificity 98.4%) when used for screening of asymptomatic persons.

What are the implications for public health practice?

To account for reduced antigen test accuracy, confirmatory testing with a nucleic acid amplification test (e.g., RT-PCR) should be considered after negative antigen test results in symptomatic persons and positive antigen test results in asymptomatic persons.

For More Information: https://www.cdc.gov/mmwr/volumes/69/wr/mm695152a3.htm

Interim Guidance for Antigen Testing for SARS-CoV-2

Authors: CDC

Summary of Recent Changes

Updates as of May 13, 2021

  • Updated guidance based on new published studies on antigen test performance.
  • Clarification about which NAATs should be used for confirmatory testing.
  • Considerations for people who have had previous SARS-CoV-2 infections and those who have been fully vaccinated.
  • Two new antigen testing algorithms, one for congregate living settings, and one for community settings.
  • Updates to testing suggestions for fully vaccinated, asymptomatic people.
    View Previous Updates

Key Points

  • This interim guidance is intended for healthcare providers who order antigen tests, receive antigen test results, or perform point-of-care testing, as well as for laboratory professionals who perform antigen testing in a laboratory setting or at the point of care and report those results.
  • The purpose of this interim technical guidance is to support effective clinical and public health use of antigen tests for different testing situations.
  • This guidance applies to all clinical and consumer uses of antigen tests and is not specific to any particular age group.
  • This guidance incorporates considerations for fully vaccinated people and should be used in conjunction with CDC’s Interim Public Health Recommendations for Fully Vaccinated People.

For More Information: https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html