What Can We Infer About a Highly Infectious Virus?
Explore the scientific framework used to evaluate viral transmission and understand the key principles that inform our collective response to infectious diseases.
Hearing that a virus is “highly infectious” can be alarming, as it suggests a pathogen that spreads with ease. The term scientifically describes a virus that efficiently moves from one individual to another, leading to rapid increases in cases. Understanding the factors behind this classification helps explain the science of epidemiology and the reasoning behind public health guidance.
Measuring Viral Spread
To quantify how infectious a virus is, scientists use metrics that describe its transmission dynamics. The basic reproduction number, or R0 (R-naught), represents the average number of new infections a single case will cause in a population with no prior immunity. For example, an R0 of 2 means one sick person is expected to infect two others. An R0 greater than 1 indicates the virus will spread, while an R0 below 1 suggests the outbreak will end.
Another measurement is the serial interval, the time between the onset of symptoms in an infected person and the person they infected. A shorter serial interval means the virus spreads more quickly through a community. For instance, studies of the SARS-CoV-2 Omicron variant found a mean serial interval of about 2.9 days. This metric, combined with R0, helps forecast an epidemic’s speed and scale.
The secondary attack rate measures transmissibility in specific settings, such as households. This rate is the probability that a susceptible person will become infected after contact with an infected individual. It is calculated by dividing the number of new cases among close contacts by the total number of those contacts.
Mechanisms of Rapid Transmission
A primary factor in viral spread is the mode of transmission, or how the virus travels between hosts. Viruses can be transmitted through large respiratory droplets expelled when an infected person coughs or sneezes. Transmission can also occur through contact with contaminated surfaces, known as fomites, though this is not the main route for many respiratory viruses.
A more efficient transmission mode is through aerosols, smaller particles that can remain suspended in the air for longer periods and travel greater distances. This airborne transmission is particularly effective in indoor environments with poor ventilation. The ability of a virus to remain viable in aerosols for hours allows it to infect people who are not in close proximity to the source, significantly increasing its infectiousness.
Viral load, the amount of virus an infected person sheds, also determines infectiousness. Individuals with a high viral load in their respiratory tract are more likely to transmit the virus. This load is often highest around the time symptoms appear but can also be substantial before symptoms are present. This presymptomatic transmission, along with asymptomatic transmission from individuals who never develop symptoms, is a driver of pandemics.
Infectiousness Versus Virulence
A common point of confusion is the distinction between how infectious a virus is (its infectiousness) and how sick it makes people (its virulence). These two characteristics are not the same and are not always linked. Infectiousness refers to the ease of transmission, while virulence describes the severity of the disease. A virus can be highly infectious but have low virulence, causing widespread but mild illness.
The common cold is a classic example of a virus with high infectiousness but low virulence, as it spreads easily but rarely causes serious illness. Conversely, a virus can have low transmissibility but high virulence, causing severe disease in the few people it infects. Some dangerous pathogens do not transmit efficiently between humans, limiting their pandemic potential.
The relationship between these two factors is complex. A virus that is extremely virulent and quickly incapacitates or kills its host may have fewer opportunities to spread, limiting its transmission. From an evolutionary perspective, there is often a trade-off, where viruses that cause less severe illness may be more successful at spreading because their hosts remain active and in contact with others.
Implications for Public Health Strategy
A virus’s infectiousness directly informs public health strategies to control its spread. For a highly infectious virus with a high R0 and airborne capabilities, interventions are designed to interrupt transmission. Measures like wearing masks, physical distancing, and improving indoor ventilation reduce the probability that the virus can travel between people. The goal of these non-pharmaceutical interventions is to lower the effective reproduction number (Rt), the real-time measure of spread in a population with some immunity and public health measures in place.
Widespread vaccination is a strategy to combat a highly infectious virus. Vaccines train the immune system to fight the pathogen, reducing the number of susceptible individuals in a population. When a high percentage of the population is immune, either through vaccination or prior infection, it creates community immunity. This makes it more difficult for the virus to find susceptible hosts and spread, thereby protecting those who cannot be vaccinated. The threshold for community immunity varies; measles requires about 95% of the population to be vaccinated to prevent its spread.