As the virus adapts, understanding how effectively these new variants spread among people is important for public health. This article explores the meaning of contagiousness, the characteristics of current prevalent variants, the biological reasons behind their increased spread, and practical measures to reduce personal exposure.
What Contagiousness Means
Contagiousness describes how easily a pathogen spreads from one infected individual to others. A key measure is the basic reproduction number, R0 (R-naught). R0 represents the average number of new infections expected to result from one infected person in a population where everyone is susceptible and no preventive measures are in place. If R0 is greater than one, the disease will spread.
Several factors influence a virus’s contagiousness. The duration an infected person remains infectious plays a role, as a longer period allows for more opportunities for transmission. The contact rate, which is the number of people an infected individual interacts with, also affects spread. Additionally, the mode of transmission, such as airborne particles or close contact, significantly impacts how readily a virus moves between hosts.
The incubation period, the time between exposure and symptom onset, also contributes to spread. Viral shedding refers to the release of infectious viral particles from an infected host. For many respiratory viruses, including COVID-19, individuals can shed the virus and be contagious before symptoms appear.
How Current Variants Spread
Current predominant COVID-19 variants, such as the “Stratus” (XFG) variant, demonstrate high levels of transmissibility. As of August 2025, Stratus accounts for a significant proportion of virus variants detected in wastewater in the United States, indicating its widespread presence. This variant, like its predecessors JN.1 and KP.3, is an Omicron sublineage known for its efficient spread.
The JN.1 variant, dominant in late 2023 and early 2024, spread efficiently and quickly became the main global variant, showing heightened transmissibility. Similarly, the KP.3 variant, which was the most common strain in the U.S. in early June 2024, is also highly contagious.
These variants primarily spread through airborne droplets released when an infected person coughs, sneezes, or talks. Close contact with an infected individual, especially indoors, increases transmission chances. A significant challenge in controlling spread comes from asymptomatic carriers, who can unknowingly pass the virus to others without exhibiting symptoms themselves.
Why Variants Become More Contagious
Viruses undergo constant genetic changes through mutation, a natural part of their evolution. These mutations can alter the virus’s characteristics, including its ability to spread. For SARS-CoV-2, changes to the spike protein, which the virus uses to enter human cells, can lead to increased infectivity.
Some mutations enable variants to bind more effectively to human cells, making it easier for the virus to initiate an infection. Other mutations can contribute to immune evasion, meaning the virus becomes better at circumventing existing immunity from previous infections or vaccinations. This allows the virus to infect individuals with existing immunity.
A shorter incubation period also contributes to increased contagiousness. For example, the Omicron variant and its sublineages typically have an incubation period of around three to four days, which is shorter than the average of 5.6 days for earlier strains. A quicker onset of symptoms, or even contagiousness before symptoms, means the virus can spread faster through a population.
Steps to Limit Exposure
To reduce the risk of exposure to highly contagious COVID-19 variants, several practical measures can be adopted. Staying up to date with COVID-19 vaccinations remains an important strategy, as vaccines significantly lower the risk of severe illness, hospitalization, and death. Even if vaccinated individuals contract the virus, their symptoms are often milder.
Wearing high-quality masks, particularly in crowded or indoor settings, helps reduce the spread of respiratory droplets. Improving indoor air quality through ventilation, such as opening windows or using portable high-efficiency particulate air (HEPA) cleaners, can further reduce airborne viral particles. Regular handwashing with soap and water or using alcohol-based hand sanitizer is also important.
Avoid touching your face, eyes, nose, and mouth with unwashed hands to prevent virus transfer. If experiencing symptoms, staying home helps prevent further transmission. Testing for COVID-19 can help confirm an infection and guide decisions on isolation and treatment, especially for those at higher risk of severe illness.