Influenza B spreads primarily through respiratory droplets and tiny airborne particles released when an infected person breathes, talks, coughs, or sneezes. These droplets can travel approximately 6 feet through the air, but smaller aerosol particles can linger in indoor spaces even longer. You can also pick up the virus by touching a contaminated surface and then touching your mouth, nose, or eyes.
Droplets, Aerosols, and Why Both Matter
For years, large respiratory droplets were considered the main route of flu transmission. These are the heavier particles produced by coughing and sneezing that fall to the ground within about 6 feet. That’s where the familiar “stay 6 feet apart” guidance comes from.
But research has shown the picture is more complex. A study published in the Proceedings of the National Academy of Sciences found that people with confirmed influenza infections shed infectious virus in fine aerosol particles (5 micrometers or smaller) just from normal breathing. Researchers recovered live, culturable virus from 39% of fine aerosol samples collected during 30 minutes of ordinary tidal breathing. Critically, sneezing was rare among participants, and coughing wasn’t required to generate these infectious aerosols. Even in breathing sessions where no coughs were observed, 48% still contained culturable virus in the fine aerosol fraction.
This matters because fine aerosols behave differently from large droplets. They’re small enough to remain suspended in the air for extended periods, meaning you don’t necessarily need to be standing right next to a sick person to be exposed. Poorly ventilated indoor spaces pose the greatest risk for this type of transmission.
When an Infected Person Is Contagious
Most adults with influenza B are infectious starting one day before symptoms appear and continuing for roughly 5 to 7 days after symptom onset. That pre-symptomatic day is especially important because people are unknowingly spreading the virus while feeling perfectly fine.
Children tend to shed the virus for longer than adults, which is one reason schools and daycares are common hotspots for influenza B outbreaks. Young children also tend to be less diligent about hand hygiene and covering coughs, compounding the problem.
Asymptomatic transmission is real but less efficient. A population cohort study in South Africa found that roughly half of all influenza infections produced no symptoms at all. Asymptomatic individuals did transmit the virus to household contacts, but at about half the rate of people with two or more symptoms. Still, 6% of household contacts exposed to an asymptomatic carrier became infected, which adds up quickly across a population.
Surface Contact and Fomite Transmission
Influenza B can also spread indirectly. When an infected person coughs or sneezes, virus-laden droplets land on nearby surfaces like doorknobs, phones, light switches, and countertops. If you touch one of these surfaces and then touch your face, the virus can enter through your mucous membranes. While this route is considered less efficient than direct respiratory transmission, it still contributes to spread, particularly in households and workplaces where people share objects frequently.
The virus survives longer on hard, non-porous surfaces like stainless steel and plastic than on porous materials like fabric and paper. Regular cleaning of high-touch surfaces during flu season reduces this risk.
Why Flu Spreads More in Winter
Influenza B follows a strong seasonal pattern, peaking in colder months. The explanation comes down to moisture in the air. Research published in PNAS found that absolute humidity, the total amount of water vapor in the air, is the strongest environmental predictor of both virus survival and transmission efficiency. Absolute humidity alone explained 90% of the variability in virus survival and 50% of the variability in transmission rates in laboratory studies.
In cold winter air, absolute humidity drops sharply, both outdoors and inside heated buildings. Under these dry conditions, respiratory droplets evaporate faster, shrinking into smaller particles that stay airborne longer. The virus itself also survives longer at low humidity. Studies consistently show the highest virus survival rates at low humidity, with some evidence suggesting a moderate humidity range of 40% to 60% is least favorable for the virus. This is one reason humidifying indoor air during winter may offer some protective benefit.
Victoria vs. Yamagata Lineage Spread
Influenza B circulates as two distinct lineages: Victoria and Yamagata. While both spread through the same mechanisms, they don’t spread at the same speed. The Victoria lineage has an effective reproduction number of 1.13 to 1.27, meaning each infected person passes the virus to roughly 1.1 to 1.3 others on average. The Yamagata lineage spreads more slowly, with an effective reproduction number of 1.08 to 1.14. In practical terms, Victoria tends to cause larger, faster-moving outbreaks. Notably, Yamagata has not been detected in global surveillance since early 2020, leading many health authorities to consider it functionally extinct.
Reducing Your Risk of Transmission
Handwashing is one of the simplest and most effective defenses. The CDC estimates that regular handwashing reduces respiratory infections by 16% to 21% in the general population. Soap and water for at least 20 seconds is the gold standard, though alcohol-based hand sanitizer works when a sink isn’t available.
Because influenza B can spread through normal breathing in enclosed spaces, ventilation matters more than most people realize. Opening windows, using air purifiers with HEPA filters, and avoiding crowded indoor spaces during peak flu season all lower exposure risk. Masks, particularly well-fitting respirators, reduce both droplet and aerosol transmission.
Annual flu vaccination remains the most effective population-level tool. Because you’re contagious a full day before you feel sick, and because a meaningful fraction of infections produce no symptoms at all, relying on staying home when ill only addresses part of the transmission chain. Vaccination reduces the likelihood that you become infected and shed virus in the first place.