https://www.thelancet.com/journals/langlo/article/PIIS2214-109X(21)00141-8/fulltext
Findings
Interpretation
https://www.jwatch.org/fw108600/2014/03/17/most-flu-asymptomatic
Most Flu Is Asymptomatic
By Joe Elia
Influenza, whether of the seasonal or pandemic variety, is asymptomatic in most people with serologically confirmed infection, according to a study in the Lancet Respiratory Medicine.
Researchers studied five successive cohorts of people in England during the 2006-2011 flu seasons. The cohorts ranged in size from 600 to 3500, and all members provided blood samples before and after each flu season. Their households were contacted weekly to identify flu-like illness and symptoms.
On average, roughly 20% of the unvaccinated had serologic evidence of influenza infection, but up to three quarters of the infected were asymptomatic. The proportions did not vary significantly between seasonal and pandemic influenzas. The pandemic H1N1 strain was associated with less severe symptoms than the seasonal H3N2 strain.
A commentator says an important unanswered question is how much the asymptomatic cases contribute to flu transmission
https://www.cdc.gov/flu/about/keyfacts.htm
The commonly cited 5% to 20% estimate was based on a study that examined both symptomatic and asymptomatic influenza illness, which means it also looked at people who may have had the flu but never knew it because they didn’t have any symptoms. The 3% to 11% range is an estimate of the proportion of people who have symptomatic flu illness.
https://hhma.org/can-the-flu-be-asymptomatic/
However, did you know that one out of every three individuals with seasonal flu infections may be asymptomatic? This is thought to be caused by pre-existing partial immunity. Certain individuals have immune systems that shed the virus and transmit the disease, but not at the same rate as patients who go on to develop symptoms. The way that certain people’s immune systems handle flu may allow them to fight the disease without displaying typical flu symptoms.
Because it is still possible for asymptomatic individuals to transmit flu, it is always important to take precautions and practice good hygiene. Cover your mouth with your elbow when you cough or sneeze, and wash your hands regularly with soap and warm water. Do your best to avoid touching your face when your hands have not just been washed.
