Incubation period is the time from initial exposure to a pathogen until the first sign of symptoms. Infectious period is the time during which a person is contagious and able to transmit an infection to other people.

What is Incubation Period?

Definition:

The incubation period is the time interval from when a person is initially exposed to a pathogen to when the symptoms of the infection first manifest. The term incubation period is usually reserved and used by epidemiologists to discuss infectious diseases, particularly those caused by viruses.

Determined by:

The incubation time for a specific virus is determined by a number of factors. Incubation time of a virus can be impacted by temperature.  Other factors that play a role in determining the incubation time include how quickly the virus replicates and how strong the host’s immune system response is. It is only after the pathogen has copied itself and its genetic material that symptoms start to appear. 

Examples:

Incubation varies from one pathogen to another.  For instance, the incubation time for the influenza virus is often only 1 day while measles is about 12 days. For COVID-19, the incubation period ranges from 11 to 14 days. For some illnesses, incubation period is very long, for example, the incubation period for hepatitis A is about  4 weeks on average, and chickenpox is from about 2 to 3 weeks.

Significance of incubation period:

Knowledge of incubation period of infectious diseases is important because it provides insight into how well control measures are working and what sort of quarantine period is needed. Incubation period can be confused with infectious period but the difference is that many times a person is not infectious while in the incubation stage of a virus. This, however, is not always the case, and asymptomatic spread in COVID-19 means that there is some infectiousness even during incubation.

What is Infectious Period?

Definition:

The infectious period is the time interval when a person can first transmit an infection to other people to when they are no longer contagious. An illness in which there is a longer infectious period is more likely to persist for longer in a population as there are more chances for contagion and spread.

Determined by:

How long a person remains infectious for depends on viral load and shedding time of the virus. There has to be enough viral particles present for the likelihood of contagion to be present. The infected person’s immune response also may determine infectious period since a strong immune response may rid the body of a virus sooner than usual.

Examples:

The infectious period of some viruses such as influenza is about 3 to 7 days while others, such as COVID-19 can be at least 14 days. Measles is a very contagious infection which is why the WHO suggests infectious period can be as long as 23 days from first contact with the virus. Infectious period of COVID-19 is longer than initially thought because asymptomatic transmission is now believed to occur.

Significance of infectious period:

Experimental studies suggest that illnesses where infectious period is long will result in greater likelihood of contagion because more contacts between infected and non-infected individuals will occur. The same study also showed that a short infectious period means lower transmission of an infection, but the drawback of this was that a newly-emerging illness may go undetected for some time due to not many infected individuals being present. Infectious period is important to know for appropriate control measures such as isolation and quarantine to be instituted. 

Difference between Incubation period and Infectious period?

Definition

The incubation period is the time between when a person is first infected to the time when symptoms first begin. The infectious period is the time between when a person is first able to transmit the infection to others to when they are no longer contagious.

Person is contagious

A person is sometimes contagious during the incubation period, but not always. A person is always contagious during the infectious period.

Virus life cycle

In the case of the incubation period, the virus life cycle includes the entry of the virus into host cells and the start of replication. In the case of the infectious period, the viral replication occurs but assembly of proteins forming the viral capsid also occurs and virions are secreted into body fluids.

Determined by

Incubation period is determined by such factors as temperature, how quickly the virus replicates, and the host’s immune response. Infectious period is determined largely by the viral load and shedding of the virus from the person, as well as the effectiveness of the host’s immune response.

Examples

Incubation period is variable from 1 day for influenza to 12 days for measles, and can be as much as 14 days for COVID-19. Infectious period also varies and for instance, is up to 7 days for influenza, 23 days for chickenpox and at least 14 days for COVID-19.

Table comparing Incubation period and Infectious period

Summary of Incubation period Vs. Infectious period

• Incubation period refers to the time from when a virus enters a host to when symptoms become apparent.
• Infectious period is the time during which a person is contagious and can spread an infectious disease.
• Both incubation and infectious period are related and are important to know for establishing the appropriate control measures for a contagious viral disease.

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