Fully vaccinated but COVID-19 positive? 5 notes on breakthrough case prevalence

Of the more than 80 million people fully vaccinated in the U.S., a tiny portion are developing breakthrough COVID-19 infections. While experts have confirmed these cases are rare, their exact incidence rate is still unclear, reports The Washington Post.

The CDC has found a small proportion of Americans — about 5,800 out of nearly 77 million — have contracted COVID-19 after being fully vaccinated against the infection, according to CNN.

Certain factors thwart fully understanding breakthrough case prevalence. Fully vaccinated individuals often forgo COVID-19 testing, and reports on breakthrough cases lag and may not reflect the most current events.

The number of breakthrough cases identified by select states does not have Anthony Fauci, MD, President Joe Biden's chief medical adviser for the pandemic, overly concerned. 

"That number of individuals who were breakthrough infections is not at all incompatible with a 90-plus percent vaccine efficacy," Dr. Fauci said at an April 9 White House news briefing. "I don't think that there needs to be concern about any shift or change in the efficacy of the vaccine." 

Here's what we know:

1. Breakthrough infections are rare, but not unexpected. No COVID-19 vaccine receiving emergency authorization from the FDA is 100 percent effective at preventing COVID-19 infection. Moderna's vaccine was 94.1 percent effective in a 30,000-person trial, and Pfizer's was 95 percent effective in a 38,000-person trial. The Johnson & Johnson vaccine was 100 percent effective at preventing COVID-19 hospitalization and death, 85 percent effective at protecting against severe cases and 72 percent effective at preventing moderate illness in U.S. trials, according to The Washington Post. Comparing the three vaccines' efficacy rates is challenging because the trials were conducted at different times during the pandemic, and in different countries with different variants and transmission rates, experts told the Post. 

2. State data underscores the rarity of such cases. For example, Michigan has reported 246 infections and three deaths between January and March among 1.7 million people fully vaccinated, according to the Detroit Free Press. Washington state officials identified 102 breakthrough cases among more than 1 million people vaccinated between February and March 20. Of these cases, most had mild symptoms, eight people required hospitalization and two people, who both lived in long-term care facilities, died, the Post reported.

At the end of March, Minnesota health officials reported 89 COVID-19 infections in fully vaccinated residents. None of the 89 breakthrough infections account for any of the state's COVID-19 deaths. On March 24, Minnesota reported 1,454,834 residents had received at least one vaccine dose, with around 800,000 Minnesotans fully vaccinated. Based on the 800,000 estimate, the 89 known cases would equal about one infection per every 9,000 vaccinated.

3. Most people who experience breakthrough infections do not have severe cases, experts say. "The ones who are sick enough to warrant hospitalization, you can count them on one hand," Nicholas Gilpin, DO, medical director for infection prevention at Southfield, Mich.-based Beaumont Health, told the Post. His observation is based on Beaumont's efforts to track breakthrough infections among 161,000 vaccinated patients. 

Early research supports Dr. Gilpin's observation. A study published March 23 in The New England Journal of Medicine found seven of 14,990 fully vaccinated employees across UC San Diego Health and the David Geffen School of Medicine at UCLA contracted COVID-19 two or more weeks after their second dose. Most of the study participants who developed a COVID-19 infection after being fully vaccinated had no symptoms or mild symptoms, indicating the vaccines were protecting against severe disease, according to Francesca Torriani, MD, lead study author and an infectious disease specialist at UC San Diego Health.

The positivity rate for healthcare workers who contracted the virus two or more weeks after vaccination was approximately .05 percent in the UC study, which was active for people vaccinated from December 16, 2020 through February 9, 2021. This rate is higher than those reported in authorized COVID-19 vaccine trials. Researchers said numerous factors could explain the elevated risk, including a regional surge at the time, regular access to testing for symptomatic and asymptomatic workers, and higher overall exposure to the virus than participants in clinical trials.

4. The reason breakthrough infections occur in rare instances is still unclear. Older people or those with compromised immune systems may generate weaker immune responses to the vaccine, putting them at higher risk for breakthrough cases, Dr. Fauci said during the April 9 White House briefing

"When someone is already elderly and may or may not have an underlying condition, that it is unfortunate, but not surprising that you might have a couple of deaths within that more than 200 people who broke through," Dr. Fauci said. 

Breakthrough cases reported to the CDC occurred among people of all ages eligible for vaccination, though more than 40 percent occurred in people 60 or older, the agency said. Sixty-five percent of cases were female and 29 percent of the infections were asymptomatic.  

5. Experts are still working to evaluate the effectiveness of the vaccines on COVID-19 variants. Early evidence suggests four of the CDC's five "variants of concern" show minimal to moderate reduction in neutralization by antibodies generated during previous infection or vaccination.

Limitations to data are further compounded by the lack of information regarding variants' role in breakthrough infections. 

"People with breakthrough infections really need to be studied," Eric Topol, MD, founder and director of La Jolla, Calif.-based Scripps Research Translational Institute, told The New Yorker. "The highest priority is to sequence the virus of the breakthrough infections."  

However, vaccinated individuals are not likely to shed enough virus to sequence, meaning that breakthrough cases may pass under the radar, further complicating the effort to understand their incidence rate. 

More articles on public health:
Week in review: 14 COVID-19 stats to know
1 in 4 American adults don't want a COVID-19 vaccine: NPR
COVID-19 death rates by state: April 12 


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