COVID-19 vs Seasonal Influenza A and B : Comparison of Clinical Features

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As the fall approaches, pediatric hospitals will start seeing children with seasonal influenza A and B. At the same time, COVID-19 will be co-circulating in communities with the flu and other respiratory viruses, making it more difficult to identify and prevent the novel coronavirus.

With little published data directly comparing the clinical features of children with COVID-19 to those with seasonal flu, researchers at Children’s National Hospital decided to conduct a retrospective cohort study of patients in the two groups.

Their findings – published September 8 in JAMA Network Open – surprised them.

The study – detailed in the article “Comparison of Clinical Features of US Children With COVID-19 vs Seasonal Influenza A and B” – showed no statistically significant differences in the rates of hospitalization, admission to the intensive care unit and mechanical ventilator use between the two groups.

The other unexpected finding was that more patients with COVID-19 than those with seasonal influenza reported fever, cough, diarrhea or vomiting, headache, body ache or chest pain at the time of diagnosis, says Xiaoyan Song, Ph.D., M.Sc., M.B., the study’s principal investigator.

“I didn’t see this coming when I was thinking about doing the study,” says Dr. Song, director of Infection Control and Epidemiology at Children’s National since 2007 and a professor of pediatrics at the George Washington University School of Medicine and Health Sciences.

“It took several rounds of thinking and combing through the data to convince myself that this was the conclusion.”

Given that much remains unknown about COVID-19, the researchers’ discovery that children with the disease present with more symptoms at the time of diagnosis is a valuable one.

“It’s a good cue from a prevention and planning perspective,” says Dr. Song. “We always emphasize early recognition and early isolation with COVID. Having a clinical picture in mind will assist clinicians as they diagnose patients with symptoms of the coronavirus.”

The study included 315 children who were diagnosed with a laboratory-confirmed COVID-19 between March 25, 2020, and May 15, 2020, and 1,402 children who were diagnosed with a laboratory-confirmed seasonal influenza between Oct. 1, 2019, and June 6, 2020, at Children’s National.

Asymptomatic patients who tested positive for COVID-19 during pre-admission or pre-procedural screening were excluded from the study.

Of the 315 patients who tested positive for COVID-19, 52% were male, with a median age of 8.4 years. Of these patients, 54 (17.1 %) were hospitalized, including 18 (5.7%) who were admitted to the intensive care unit (ICU) and 10 (3.2%) who received mechanical ventilator treatment.

Among the 1,402 patients who tested positive for influenza A or B, 52% were male, with a median age of 3.9 years, and 291 (21.2%) were hospitalized, including 143 for influenza A and 148 for influenza B. Ninety-eight patients (7.0%) were admitted to the ICU, and 27 (1.9%) received mechanical ventilator support.

The study showed a slight difference in the age of children hospitalized with COVID-19 compared to those hospitalized with seasonal influenza. Patients hospitalized with COVID-19 had a median age of 9.7 years vs. those hospitalized with seasonal influenza who had a median age of 4.2 years.

In both groups, fever was the most often reported symptom at the time of diagnosis followed by cough. A greater proportion of patients hospitalized with COVID-19 than those hospitalized with seasonal influenza reported fever (76% vs. 55%), cough (48% vs. 31%), diarrhea or vomiting (26% vs. 12%), headache (11% vs. 3%), body ache/myalgia (22% vs. 7%), and chest pain (11% vs. 3%).

More patients hospitalized with COVID-19 than those with seasonal influenza reported sore throat or congestion (22% vs. 20%) and shortness of breath (30% vs. 20%), but the differences were not statistically significant.

During the study period, the researchers noticed an abrupt decline of influenza cases at Children’s National after local schools closed in mid-March and stay-at-home orders were implemented about two weeks later to combat the community spread of COVID-19. Dr. Song says the impact of school closures on the spread of COVID-19 among children is the next area of study for her research team.

“We want to assess the quantitative impact of school closures so we can determine at what point the cost of closing schools and staying at home outweighs the benefit of reducing transmission of COVID-19 and burdens on the health care system,” she says.

Dr. Song urges members of the community “first and foremost to stay calm and be strong. We’re learning new and valuable things about this virus each day, which in turn improves care.

The collision of the flu and COVID-19 this fall could mean an increase in pediatric hospitalizations. That’s why it’s important to get your flu shot, because it can help take at least one respiratory virus out of circulation.”


Signs and Symptoms

Similarities:

Both COVID-19 and flu can have varying degrees of signs and symptoms, ranging from no symptoms (asymptomatic) to severe symptoms. Common symptoms that COVID-19 and flu share include:

  • Fever or feeling feverish/chills
  • Cough
  • Shortness of breath or difficulty breathing
  • Fatigue (tiredness)
  • Sore throat
  • Runny or stuffy nose
  • Muscle pain or body aches
  • Headache
  • Some people may have vomiting and diarrhea, though this is more common in children than adults

Differences:

Flu

Flu viruses can cause mild to severe illness, including common signs and symptoms listed above.

Flu Symptoms

Influenza (flu) can cause mild to severe illness, and at times can lead to death. Flu is different from a cold. Flu usually comes on suddenly. People who have flu often feel some or all of these symptoms:

  • fever* or feeling feverish/chills
  • cough
  • sore throat
  • runny or stuffy nose
  • muscle or body aches
  • headaches
  • fatigue (tiredness)
  • some people may have vomiting and diarrhea, though this is more common in children than adults.

*It’s important to note that not everyone with flu will have a fever

This video includes common signs and symptoms of flu.

Flu Complications

Most people who get flu will recover in a few days to less than two weeks, but some people will develop complications (such as pneumonia) as a result of flu, some of which can be life-threatening and result in death.

Sinus and ear infections are examples of moderate complications from flu, while pneumonia is a serious flu complication that can result from either influenza virus infection alone or from co-infection of flu virus and bacteria. Other possible serious complications triggered by flu can include inflammation of the heart (myocarditis), brain (encephalitis) or muscle (myositis, rhabdomyolysis) tissues, and multi-organ failure (for example, respiratory and kidney failure).

Flu virus infection of the respiratory tract can trigger an extreme inflammatory response in the body and can lead to sepsis, the body’s life-threatening response to infection. Flu also can make chronic medical problems worse.

For example, people with asthma may experience asthma attacks while they have flu, and people with chronic heart disease may experience a worsening of this condition triggered by flu.

People at High Risk from Flu

Anyone can get sick with flu (even healthy people), and serious problems related to flu can happen at any age, but some people are at high risk of developing serious flu-related complications if they get sick.

This includes people 65 years and older, people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women and children younger than 5 years, but especially those younger than 2 years old.What are the emergency warning signs of flu?

People experiencing these warning signs should obtain medical care right away.

In children

  • Fast breathing or trouble breathing
  • Bluish lips or face
  • Ribs pulling in with each breath
  • Chest pain
  • Severe muscle pain (child refuses to walk)
  • Dehydration (no urine for 8 hours, dry mouth, no tears when crying)
  • Not alert or interacting when awake
  • Seizures
  • Fever above 104°F
  • In children less than 12 weeks, any fever
  • Fever or cough that improve but then return or worsen
  • Worsening of chronic medical conditions

In adults

  • Difficulty breathing or shortness of breath
  • Persistent pain or pressure in the chest or abdomen
  • Persistent dizziness, confusion, inability to arouse
  • Seizures
  • Not urinating
  • Severe muscle pain
  • Severe weakness or unsteadiness
  • Fever or cough that improve but then return or worsen
  • Worsening of chronic medical conditions

These lists are not all inclusive. Please consult your medical provider for any other symptom that is severe or concerning.

Signs and SymptomsColdInfluenza (Flu)
Symptom onsetGradualAbrupt
FeverRareUsual; lasts 3-4 days
AchesSlightUsual; often severe
ChillsUncommonFairly common
Fatigue, weaknessSometimesUsual
SneezingCommonSometimes
Chest discomfort, coughMild to moderate; hacking coughCommon; can be severe
Stuffy noseCommonSometimes
Sore throatCommonSometimes
HeadacheRareCommon

Symptoms of Coronavirus

What you need to know

  • Anyone can have mild to severe symptoms.
  • Older adults and people who have severe underlying medical conditions like heart or lung disease or diabetes seem to be at higher risk for developing more serious complications from COVID-19 illness.

Watch for symptoms

People with COVID-19 have had a wide range of symptoms reported – ranging from mild symptoms to severe illness. Symptoms may appear 2-14 days after exposure to the virus. People with these symptoms may have COVID-19:

  • Fever or chills
  • Cough
  • Shortness of breath or difficulty breathing
  • Fatigue
  • Muscle or body aches
  • Headache
  • New loss of taste or smell
  • Sore throat
  • Congestion or runny nose
  • Nausea or vomiting
  • Diarrhea

This list does not include all possible symptoms. CDC will continue to update this list as we learn more about COVID-19.When to seek emergency medical attention

Look for emergency warning signs* for COVID-19. If someone is showing any of these signs, seek emergency medical care immediately:

  • Trouble breathing
  • Persistent pain or pressure in the chest
  • New confusion
  • Inability to wake or stay awake
  • Bluish lips or face

*This list is not all possible symptoms. Please call your medical provider for any other symptoms that are severe or concerning to you.

Call 911 or call ahead to your local emergency facility: Notify the operator that you are seeking care for someone who has or may have COVID-19.

How are COVID-19 and influenza viruses similar?

Firstly, COVID-19 and influenza viruses have a similar disease presentation. That is, they both cause respiratory disease, which presents as a wide range of illness from asymptomatic or mild through to severe disease and death.

Secondly, both viruses are transmitted by contact, droplets and fomites. As a result, the same public health measures, such as hand hygiene and good respiratory etiquette (coughing into your elbow or into a tissue and immediately disposing of the tissue), are important actions all can take to prevent infection. How are COVID-19 and influenza viruses different?

The speed of transmission is an important point of difference between the two viruses. Influenza has a shorter median incubation period (the time from infection to appearance of symptoms) and a shorter serial interval (the time between successive cases) than COVID-19 virus.

The serial interval for COVID-19 virus is estimated to be 5-6 days, while for influenza virus, the serial interval is 3 days. This means that influenza can spread faster than COVID-19. 

Further, transmission in the first 3-5 days of illness, or potentially pre-symptomatic transmission –transmission of the virus before the appearance of symptoms – is a major driver of transmission for influenza. In contrast, while we are learning that there are people who can shed COVID-19 virus 24-48 hours prior to symptom onset, at present, this does not appear to be a major driver of transmission. 

The reproductive number – the number of secondary infections generated from one infected individual – is understood to be between 2 and 2.5 for COVID-19 virus, higher than for influenza. However, estimates for both COVID-19 and influenza viruses are very context and time-specific, making direct comparisons more difficult.  

Children are important drivers of influenza virus transmission in the community. For COVID-19 virus, initial data indicates that children are less affected than adults and that clinical attack rates in the 0-19 age group are low.

Further preliminary data from household transmission studies in China suggest that children are infected from adults, rather than vice versa.

While the range of symptoms for the two viruses is similar, the fraction with severe disease appears to be different. For COVID-19, data to date suggest that 80% of infections are mild or asymptomatic, 15% are severe infection, requiring oxygen and 5% are critical infections, requiring ventilation. These fractions of severe and critical infection would be higher than what is observed for influenza infection.

Those most at risk for severe influenza infection are children, pregnant women, elderly, those with underlying chronic medical conditions and those who are immunosuppressed. For COVID-19, our current understanding is that older age and underlying conditions increase the risk for severe infection.

Mortality for COVID-19 appears higher than for influenza, especially seasonal influenza. While the true mortality of COVID-19 will take some time to fully understand, the data we have so far indicate that the crude mortality ratio (the number of reported deaths divided by the reported cases) is between 3-4%, the infection mortality rate (the number of reported deaths divided by the number of infections) will be lower. For seasonal influenza, mortality is usually well below 0.1%. However, mortality is to a large extent determined by access to and quality of health care. 


More information: JAMA Network Open (2020). DOI: 10.1001/jamanetworkopen.2020.20495

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