4 Theories On Why So Many Coronavirus Cases Are Asymptomatic

Your Choice Regarding Extending Session on this Site

You may be receiving this error message if you are logged out. Be sure to log into your account to access the resources.

Auto logout in seconds.

Continue Logout

Today's Updates

• VBC success is possible. Here's how. — Get our latest insights and exclusive case studies.

• Podcast

  Ep. 280: The questions Advisory Board is asking in 2026

• Webinar

  The state of the industry: Key insights for 2026

• Tools

  Use Advisory Board tools to inform your strategy for growth, cost control, and more.

• Timeline

  Track the latest Healthcare Policy Updates

• Daily Briefing

  Join 165,000+ healthcare leaders and get the industry’s most important news in your inbox—every day.

Previous news Next news

• Our Research

• Care Delivery

  • Ambulatory care
  • Behavioral health
  • Cardiovascular
  • Oncology
  • Pharmacy
  • Population health
  • Post-acute care
  • Service lines and specialty care
  • Weight management and obesity care
  • Women's health

  Learn more

  Strategy

  • Consumer trends
  • Finance
  • Health plan strategy
  • Healthcare policy updates
  • Hospital and health system trends
  • International
  • Market trends and growth strategy
  • Site-of-care shifts
  • State of the healthcare industry
  • Value-based care

  Staffing & Operations

  • Facility planning
  • Leadership
  • Nursing
  • Philanthropy
  • Physicians and medical groups
  • Provider operations
  • Sales enablement
  • Workforce

  Technology & Innovation

  • Artificial intelligence
  • Digital access and experience
  • Digital health
  • Medical device
  • Pharmaceutical
  • Telehealth

  LATEST RESEARCH

  

  3 trends shaping healthcare in 2026 (and how to respond)

  

  The road ahead for hospitals: Financial insights that could shape 2026

  

  What healthcare leaders are prioritizing in 2026

  More latest research

  STAY IN THE KNOW

  Daily Briefing

  A must-read daily newsletter for thousands of healthcare executives across the nation.

  Radio Advisory

  A podcast providing your weekly download on how to untangle healthcare's most pressing challenges. New episodes drop every Tuesday.

• Tools

• FORECASTING & DEMAND

  • Market Scenario Planner
  • Demographic Profiler
  • Clinician Supply Profiler
  • Cancer Incidence Estimator

  COMPETITION & MARKET SHARE

  • Medicare Market Explorer
  • Health Care Facility Quickview Map
  • Health Plan Market Explorer

  PERFORMANCE IMPROVEMENT & BENCHMARKING

  • Care Variation Reduction Assessment
  • Hospital Benchmark Generator
  • Hospital Performance Profiler

  POST-ACUTE CARE & NETWORK DESIGN

  • Post-Acute Care Pathways Explorer
  • Home Health Performance Profiler
  • Home Health Benchmark Generator
  • Skilled Nursing Facility Benchmark Generator
  • Skilled Nursing Facility Performance Profiler

  See all tools

   

   

• Events & Webinars

• ADVISORY BOARD EVENTS

  In-person and virtual experiences to learn from the best in the healthcare industry and network with your peers.

  Upcoming Events:

  • March 31 ❘ Cardiovascular Roundtable: Tactics for sustainable procedure growth ❘ Virtual
  • May 04 ❘ Advisory Board 2026 Summit: Washington, D.C. ❘ In-Person
  • June 02 ❘ Advisory Board 2026 Summit: Carlsbad, CA ❘ In-Person
  • October 13 ❘ Advisory Board 2026 Summit: Chicago, IL ❘ In-Person

  See all events

   

  WEBINARS

  Join your peers to learn the latest insights in our live and recorded webinars.

  Upcoming Webinars:

  • January 08 ❘ State of the Industry: Protecting access amid funding cuts
  • January 13 ❘ Orthopedic market trends
  • January 15 ❘ State of the Industry: Managing spend amid rising costs
  • January 27 ❘ How to embrace a proactive (not reactive) approach to change

  See all upcoming and on-demand webinars

• Products & Services
• 

  Research Membership

  Access to insights, resources, and tools designed to address the healthcare industry's most pressing challenges.

  • Research & Insights
  • Data & Analytical Planning Tools
  • Webinars
  • Expert Conversations
  

  ASKADVISORY

  Our direct-to-expert service is here to help you navigate your membership, our research, and your most pressing challenges in healthcare. No question is too big or too small.

  Custom Research

  Original research into the topics you care about most, conducted the Advisory Board way.

  Expert Support

  Connections to healthcare experts that help break through challenges and identify the most pressing opportunities.

  Advisory Board Fellowship

  An immersive leadership practice ground, empowering leaders to make progress on organizational challenges.

  Events & Roundtables

  In-person and virtual experiences designed to deliver crucial healthcare information and foster meaningful connections with peers and industry experts.

  Webinars

  Join our expert-led webinars to get the latest insights on topics such as the state of the industry, value-based care, health system strategy, specialty care market trends, and more.

  Sponsorship

  Thought leadership opportunities to elevate your brand, showcase your expertise, and engage with key decision-makers.

  See all Products & Services

• About Us

• ADVISORY BOARD

  We help leaders and future leaders in the healthcare industry work smarter and faster by providing provocative insights, actionable strategies, and practical tools to support execution.

  Learn more about us

   

   

  Who We Serve

  We empower our members, and the entire healthcare industry, to confidently reimagine healthcare with breakthrough insights that open minds and inspire action.

  OUR HISTORY

  Advisory Board has changed a lot over its 40-year history, but what has not changed is our commitment to addressing tough questions with unbiased, disciplined research.

  OUR EXPERTS

  Our team of 200+ curious researchers work full time to uncover the insights that you need to know and the solutions that will work.

  CAREERS

  Join us to improve the lives of others while doing your life's best work.

Questions? AskAdvisory

Library

| Daily Briefing

5 minute read 4 theories on why so many coronavirus cases are asymptomatic

Since the start of the coronavirus pandemic, scientists have been puzzled about why many people who contract the virus develop symptoms of Covid-19 while others don't experience any symptoms at all. Now, recent research has produced a handful of theories, Ariana Eunjung Cha reports for the Washington Post.

Cheat sheets: Population Health 101

Large numbers of asymptomatic patients

Data has shown that a large percentage of people who've been infected with the novel coronavirus do not experience symptoms of Covid-19, the disease the virus can cause. For instance, 88% of 147 infected residents at a Boston homeless shelter were asymptomatic, while 95% of the 481 people infected at a Tyson Foods factory showed no symptoms, Cha reports.

This trend is significant, Cha reports, because understanding what shielded asymptomatic individuals from severe illness could help researchers develop vaccines and therapies to treat Covid-19, or to potentially create ways to develop herd immunity against the virus.

Now, initial research is finally revealing some clues, Cha writes. Here are four theories research suggests may be the reason so many people infected with the new coronavirus are asymptomatic:

Report: Investing in the well-being of the health care workforce

1. T-cell memory

One theory suggests that some people have partial immunity to the coronavirus due to so-called "memory" T cells—white blood cells that run the immune system and are in charge of recognizing invaders, Cha reports.

For instance, researchers in a study published in the journal Cell compared blood samples from people recovering from Covid-19 with samples from uninfected people who donated blood between 2015 and 2018. According to the researchers, the T cells in 40% to 60% of the old samples appeared to recognize the new coronavirus—a finding echoed by research teams in the Netherlands, Germany, and Singapore.

A related paper published this week in the journal Science suggested that this partial immunity could come from exposure to other coronaviruses, such as those that cause the common cold. And the immune system's strongest response, according to the study, was against the spike proteins used by the novel coronavirus to break into cells—indicating "that fewer of these viral copies get past these defenses," Cha writes.

"The [new coronavirus] didn't even exist back [when the old blood samples were taken], so to have this immune response was remarkable," Alessandro Sette, one of the authors of the Science and Cell studies from the La Jolla Institute for Immunology, said. And citing the immune response to spike proteins in particular, Sette added, "The current model assumes you are either protected or you are not—that it's a yes or no thing. But if some people have some level of preexisting immunity, that may suggest it's not a switch but more continuous."

NIH Director Francis Collins said this theory could "potentially explain why some people seem to fend off the virus and may be less susceptible to becoming severely ill."

However, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, cautioned that the theory is premature, though he agreed that some partial, already present immunity in some patients is a possibility. "There are so many other unknown factors that maybe determine why someone gets an asymptomatic infection," such as age and overall health, he said. "It's a very difficult problem to pinpoint one thing."

Infographic: Past, present, and future care for an aging population

2. Immunity from childhood vaccinations

Another theory suggests that childhood vaccines may have provided partial immunity against the new coronavirus for some patients, Cha reports.

Andrew Badley, a researcher at the Mayo Clinic, and his team collaborated with data experts from nference to examine 137,037 Mayo patient records to look for a link between vaccinations and infection from the novel coronavirus.

They found that seven types of childhood vaccines—administered one, two, or five years previously—were associated with having a lower infection rate from the coronavirus. This was especially true among people who recently received a pneumonia vaccine, which was associated with a 28% reduction in coronavirus-infection risk, and polio vaccines, which were associated with a 43% reduction in coronavirus-infection risk. And those associations held even after adjusting for a variety of factors, including geographic incidence of the virus, demographics, and underlying conditions, the researchers said.

However, since the study is observational and cannot show causality, researchers at Mayo are now looking at ways to quantify how the vaccines affect the new coronavirus, Cha reports. After all, as Badley pointed out, if existing vaccines end up being as effective against the coronavirus as vaccines currently under development, the finding could upend how countries are approaching vaccines against the novel virus altogether.

Resource library: Expedite patient diagnosis

3. Biology

Researchers at NIH are looking into a theory that suggests ACE2 receptors may affect the severity of illness a person develops from the new coronavirus.

According to Cha, the coronavirus can "latch" onto ACE2 receptors, which in healthy people keep blood pressure stable, then travel through the body and replicate. Researchers are intrigued by the receptors because they've theorized that minimizing those receptors may obstruct the virus' ability to replicate or "trick the virus into attaching itself to a drug" instead, so it's not able to replicate and travel through the body, Cha reports.

In addition, counterintuitive findings suggest that "allergic reactions may protect you by down-regulating the receptor," Alkis Togias, an NIH researcher, said—although he cautioned it was "only a theory."

Specifically, recent research has found that ACE2 receptors were diminished in children with lots of allergies and asthma—findings that, when paired with hospital data, indicate that asthma doesn't appear to be a risk factor for the new coronavirus.

As a result, Togias and his team are working on a study of 2,000 American families to see how ACE2 receptors are expressed differently as people age, hoping to better understand how the receptors affect the new coronavirus by comparing the receptors' differences and immune responses within the families, Cha reports.

Cheat sheets: Evidence-based medicine 101

4. Masks

Researchers are also exploring whether widespread mask use affects the severity of Covid-19, Cha reports.

Monica Gandhi, a researcher at the University of California, noticed how different asymptomatic case numbers were on two different cruise ships. On the Diamond Princess, where masks weren't used, 47% of those infected with the virus were asymptomatic. But on an Argentine cruise ship, where all passengers were given surgical masks and crew received N95s, 81% of cases were asymptomatic. Similarly, according to Gandhi, countries that implemented population-level mask mandates—including the Czech Republic, Singapore, and Vietnam—"got cases, but fewer deaths."

And Gandhi also noted, in an article published in the Journal of General Internal Medicine, that in some early coronavirus outbreaks where people weren't wearing masks, 15% of those infected were asymptomatic. However, later in the pandemic, when more people were wearing masks, asymptomatic rates jumped between 40% and 45%. Gandhi said that data indicates that masks may not only protect others—as health officials in the United States have stated—but they may also protect the wearer (Cha, Washington Post, 8/8).

Your top resources for Covid-19 readiness

Access our resource library

Learn from the top health plan resources on how to safely manage and prevent the spread of Covid-19 with our library of research on topics from provider network support to payer strategic outlook. 

Access now

Is this content helpful?

related resources

COVID-19 variant surge toolkit

Cheat sheets: Population Health 101

Investing in the well-being of the healthcare workforce

SPONSORED BY INTENDED AUDIENCE AFTER YOU READ THIS AUTHORS

Posted on August 13, 2020

Updated on May 09, 2023

TOPICS INDUSTRY SECTORS MORE FROM TODAY'S DAILY BRIEFING Daily Briefing

What medical residents are paid—and what they really think about their bosses

The average resident salary rose about 3% from last year—but salaries varied significantly by specialty, according to Medscape's "Residents Salary & Debt Report 2020." Daily Briefing

4 lessons from Centura on providing comprehensive support for employees during Covid-19

Centura Health is taking a comprehensive approach to employee support. Advisory Board's Abby Burns spoke with Centura's Cal Paries and Di Thompson—two leaders of the health system's employee support programs—to distill four key takeaways from Centura's success. Daily Briefing

CMS has a new plan to push rural providers to value-based payments. Here's what's inside.

CMS on Tuesday announced a new alternative payment model that aims to improve rural health care and shift providers toward more value-based payments. Here's how it will work—and what it means for rural hospitals and ACOs, according to Advisory Board's Madeline Chaillet and Darby Sullivan. Daily Briefing

'Covid fog': The latest emerging, long-term symptom of Covid-19

Up to one-third of people who had Covid-19 report lingering neurological and psychological symptoms due to the disease—a finding that "reflect[s] a growing consensus that the disease can have lasting impact on the brain," Elizabeth Cooney reports for STAT News.

We help leaders and future leaders in the healthcare industry work smarter and faster by providing provocative insights, actionable strategies, and practical tools to support execution.

Research & Events

• Latest research
• Daily Briefing
• Radio Advisory
• Events
• Search

Company

• About us
• Our history
• Products
• Sponsorship
• Careers

Support

• AskAdvisory
• Contact us

Social

• Facebook
• Instagram
• LinkedIn

• Legal Disclaimer
• Privacy Policy
• Terms of Use

Don't miss out on the latest Advisory Board insights

Create your free account to access 1 resource, including the latest research and webinars.

Create Account Sign in to your account

Want access without creating an account?

   

Get Limited Access

You have 1 free members-only resource remaining this month.

View this resource

1 free members-only resources remaining

Become a Member

1 free members-only resources remaining

Create Account

You've reached your limit of free insights

Become a member to access all of Advisory Board's resources, events, and experts

Never miss out on the latest innovative health care content tailored to you.

Become a Member

Benefits include:

Unlimited access to research and resources Member-only access to events and trainings Expert-led consultation and facilitation The latest content delivered to your inbox Become a Member

You've reached your limit of free insights

Become a member to access all of Advisory Board's resources, events, and experts

Never miss out on the latest innovative health care content tailored to you.

Create Account Become a Member

Benefits include:

Unlimited access to research and resources Member-only access to events and trainings Expert-led consultation and facilitation The latest content delivered to your inbox

This content is available through your Curated Research partnership with Advisory Board. Click on ‘view this resource’ to read the full piece

View this resource

Email ask@advisory.com to learn more

Click on ‘Become a Member’ to learn about the benefits of a Full-Access partnership with Advisory Board

Never miss out on the latest innovative health care content tailored to you. 

Become a Member

Benefits Include:

Unlimited access to research and resources Member-only access to events and trainings Expert-led consultation and facilitation The latest content delivered to your inbox Become a Member

This is for members only. Learn more.

Click on ‘Become a Member’ to learn about the benefits of a Full-Access partnership with Advisory Board

Never miss out on the latest innovative health care content tailored to you. 

Become a Member

Benefits Include:

Unlimited access to research and resources Member-only access to events and trainings Expert-led consultation and facilitation The latest content delivered to your inbox Become a Member You Session Is Expiring

Your session is about to expire. Click continue to stay logged in.

Auto logout in seconds.

Continue Logout Thank you! Your updates have been made successfully. Oh no! There was a problem with your request. Error in form submission. Please try again.