People With Asymptomatic COVID-19 May Have Higher White Blood Cell Counts: Study

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Key Takeaways

  • A new study found that asymptomatic COVID-19 patients have an elevated white blood cell count compared to symptomatic people, indicating that they have better immune function.
  • About 45% of people infected with COVID-19 are asymptomatic, meaning they exhibit no symptoms.
  • Asymptomatic people account for two-thirds of COVID-19 transmissions.

By now, the hallmarks of a COVID-19 infection are world-famous: fever, chills, headache, fatigue, coughing, and difficulty breathing, among others. For reasons still unknown to medicine, however, a significant number of infected people—about 45%, according to recent estimates—are asymptomatic, meaning that they exhibit no symptoms. No fever, no chills, nothing.

It’s an epidemiological puzzle that has stumped medical professionals since the early days of the pandemic, but the results of a study conducted by researchers in Wuhan, China, provides insight into the biological mechanisms that control how our immune systems respond to the virus. This September study may have found a clue: a type of white blood cell known as a lymphocyte.

Published in the journal mSphere, the study found that while symptomatic and asymptomatic people play host to a similar viral load—a term for the number of viral particles present in blood or other bodily fluids—asymptomatic people have a much higher count of white blood cells, especially a type of lymphocyte known as CD4+ T cells. Possibly not coincidentally, asymptomatic people also recover faster, have shorter hospital stays, and are less likely to suffer long-term complications.

“Our findings suggested an important role for lymphocytes, especially T cells, in controlling virus shedding,” lead study author Yuchen Xia, PhD, professor at Wuhan University’s School of Basic Medical Sciences, told the American Society for Microbiology.

What This Means For You

Even if you don't experience symptoms, you could potentially be infected with COVID-19 and capable of passing it to others. That's why it's important to adhere to safety precautions like wearing a mask, social distancing, and hand washing.

The Findings

Controlling for age, sex, and preexisting conditions, the Wuhan researchers collected and analyzed throat swabs and blood samples from 27 symptomatic people and 25 asymptomatic people with COVID-19 who had been admitted to Renmin Hospital of Wuhan University between January 31 and April 16. The symptomatic cohort had been admitted due to complications from COVID-19, while the asymptomatic cohort had been admitted for other reasons but tested positive upon arrival.

In addition to the fact that symptomatic people had a lower lymphocyte count than asymptomatic people, the researchers found that symptomatic people had higher levels of the enzymes ALT, AST, and LDH and lower levels of the liver protein products ALB and TP than asymptomatic patients. Both sets of biomarkers are often associated with impaired liver function, which Dean Blumberg, MD, chief of pediatric infectious diseases at UC Davis Health and UC Davis Children’s Hospital in California, tells Verywell is a common symptom of COVID-19.

“Patients with COVID have systemic inflammation, often including inflammation of the liver,” he says. 

How Do White Blood Cells Work? 

White blood cells are the body’s first line of defense against invaders, whether they're viral, bacterial, or parasitic. These cells include lymphocytes, which have slightly different functions depending on their structural makeup. T cells, for example, are named for their ability to curb tumor growth, B cells have an ability to produce antibodies, and NK cells, or “natural killer” cells, are able to cause apoptosis, or cell death. 

Blumberg says that a low lymphocyte count is not necessarily a reflection of the quality of baseline immune system function.

“It is possible that these immune cells provide primary protection against disease and control the infection,” Blumberg says. “However, an alternative explanation is that SARS-CoV-2 infection results in depression of the lymphocyte count in symptomatic patients who have a more invasive infection—we often see this with viral infections. So the difference in lymphocyte counts may be the result of severe infection, and might not indicate protection in those who are ultimately asymptomatic.”

According to Blumberg, disparate factors can affect your lymphocyte count, including:

  • Age
  • Stress level
  • Medical history (including HIV infection, tumor development, and cancer treatment)

Children, for example, tend to have a higher lymphocyte count than adults, which could explain why preteens and teenagers seem to be less susceptible to COVID-19 than older adults.  

Asymptomatic Carriers Are Still Contagious

Whichever way you slice it, it’s undoubtedly preferable to be asymptomatic than symptomatic, Blumberg says. Becoming symptomatic, he says, “can result in severe disease, hospitalization, long-term effects, and even death.” Over 225,000 people in the United States and 1.1 million people worldwide, according to Johns Hopkins University’s COVID-19 tracker, have already died of COVID-19. But the fact that such a large proportion of people with COVID-19 are asymptomatic doesn’t minimize the severity of the disease; rather, it highlights the importance of safety precautions such as masking and social distancing. 

“Since asymptomatic patients have no clinical symptoms which can easily prevent timely diagnosis and treatment, they may cause a greater risk of virus transmission than symptomatic patients, which poses a major challenge to infection control,” the study authors wrote. 

According to Blumberg, asymptomatic people account for two-thirds of coronavirus transmissions—something to keep in mind when you’re interacting with people outside of your immediate household. 

“Even if you or others appear well, transmission may occur,” he says.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Oran D, Topol E. Prevalence of asymptomatic SARS-CoV-2 infection. Ann Intern Med. 2020;173(5):362-367. doi:10.7326/m20-3012

  2. Han H, Xu Z, Cheng X, et al. Descriptive, retrospective study of the clinical characteristics of asymptomatic covid-19 patients. Rosenberg HF, ed. mSphere. 2020;5(5):e00922-20, doi:10.1128//msphere/5/5/mSphere922-20.atom

  3. American Society for Microbiology. New study suggests crucial role for lymphocytes in asymptomatic COVID-19 infection.

  4. The National Human Genome Research Institute. Lymphocytes.

  5. Lin Y, Kim J, Metter EJ, et al. Changes in blood lymphocyte numbers with age in vivo and their association with the levels of cytokines/cytokine receptorsImmun Ageing. 2016;13(1):24. doi:10.1186/s12979-016-0079-7

  6. Johns Hopkins University of Medicine. Coronavirus resource center.

By Caroline Tien
Caroline Tien is a journalist with degrees in English and biology. She has previously written for publications including Insider and Cancer Health.