Latent period (epidemiology)

In epidemiology, particularly in the discussion of infectious disease dynamics (modeling), the latent period (also known as the latency period or the pre-infectious period) is the time interval between when an individual or host is infected by a pathogen and when that individual becomes infectious, i.e. capable of transmitting pathogens to other susceptible individuals.

To understand the spreading dynamics of an infectious disease or an epidemic, three important time periods should be carefully distinguished: incubation period, pre-infectious or latent period and infectious period. Two other relevant and important time period concepts are generation time and serial interval.

The infection of a disease begins when a pathogenic (disease-causing) infectious agent, or a pathogen, is successfully transmitted from one host to another. Pathogens leave the body of one host through a portal of exit, are carried by some mode of transmission and after coming into contact (exposure) with a new susceptible host, they enter the host's body through an appropriate portal of entry. Upon entering the new host, they take a period of time to overcome or evade the immune response of the body and to multiply or replicate after having traveled to their favored sites within the host’s body (tissue invasion and tropism). When the pathogens become sufficiently numerous and toxic to cause damage to the body, the host begins to display symptoms of a clinical disease (i.e. the host becomes symptomatic).

The time interval from the time of invasion by an infectious pathogen to the time of onset (first appearance) of symptoms of the disease in question is called the incubation period. After the incubation period is over, the host enters the symptomatic period. Moreover, at a certain point in time after infection, the host becomes capable of transmitting pathogens to others, i.e. they become infectious or communicable. Depending on the disease, the host individual may or may not be infectious during the incubation period. The incubation period is important in the dynamics of disease transmission because it determines the time of case detection relative to the time of infection. This helps in the evaluation of the outcomes of control measures based on symptomatic surveillance. The incubation period is also useful to count the number of infected people.

The period from the time of infection to the time of becoming infectious is called the pre-infectious period or the latent period. During the pre-infectious or latent period, a host may or may not show symptoms (i.e. the incubation period may or may not be over), but in both cases, the host is not capable of infecting other hosts i.e. transmitting pathogens to other hosts. The latent period, rather than the incubation period, has more influence on the spreading dynamics of an infectious disease or epidemic.

The time interval during which the host is infectious, i.e. the pathogens can be transmitted directly or indirectly from the infected host to another individual, is called the infectious period (or the period of communicability), defined as the period from the end of the pre-infectious period or the latent period until the time when the host can no longer transmit the infection to other individuals. During the infectious period, a host may or may not show symptoms, but they are capable of infecting other individuals. The duration of the infectious period depends on the ability of the infected host individual to mount an immune response.

In some cases, the pre-infectious or latent period and the incubation period coincide and are mostly of the same duration. In this case, the infected individual becomes infectious at around the same time they start showing symptoms. In certain other infectious diseases such as smallpox or SARS, the host becomes infectious after the onset of symptoms. In this case, the latent period is longer than the incubation period. In these two cases, the disease can be effectively controlled using symptomatic surveillance. A related term is the duration of shedding or the shedding period, which is defined as the time duration during which a host or patient excretes pathogens through saliva, urine, feces or other bodily fluids.

However, for some infectious diseases, the symptoms of the clinical disease may appear after the host becomes infectious. In this case, the pre-infectious or latent period has a shorter duration than the incubation period, the infectious period begins before the end of the incubation period and the host can infect others for some time without showing any noticeable symptoms. This early or mild stage of infection whose symptoms stay below the level of clinical detection is called subclinical infection and the individual concerned is called an asymptomatic carrier of the disease. For example, in HIV/AIDS, the incubation period lasts years longer than the latent period. So an HIV infected individual can show no symptoms and unwittingly infect other susceptible individuals for many years. In COVID-19, the infectious period begins approximately 2 days before the onset of symptoms and 44% of the secondary infections may happen during this pre-symptomatic stage. In these kinds of cases with a significant number of pre-symptomatic (asymptomatic) transmissions, symptomatic surveillance-based disease control measures (such as isolation, contact tracing, enhanced hygiene, etc.) are likely to have their effectiveness reduced, because a significant portion of the transmission may take place before the onset of symptoms and this has to be taken into account when designing control measures.