Influenza virus transmission period

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Influenza Virus Transmission Period: Key Timelines and Dynamics

Incubation Period and Serial Interval of Influenza Virus

The incubation period for human-to-human transmitted influenza A(H7N9) virus is typically around 4 days, with the serial interval (the time between symptom onset in a primary case and a secondary case) being about 3 to 9 days, depending on the strain and study population. These timelines are important for monitoring and controlling the spread of the virus, as most secondary infections occur within this window after exposure to an infected individual 12.

Infectious Period in Humans

Most healthy adults infected with influenza can transmit the virus to others starting about 1 day before symptoms appear and up to 5–7 days after becoming sick. Children, however, may remain infectious for longer than 7 days. This means that people can spread the virus even before they realize they are sick, and for several days after symptoms begin .

Experimental and Animal Model Insights

Experimental studies in mice show that the highest risk of transmission occurs within 24 to 48 hours after infection begins, even though the virus can remain at high levels in the body for longer. In pigs, which are considered a good model for human influenza, the true infectious period averages about 3.9 days, slightly shorter than the period during which the virus can be detected in nasal swabs (about 4.5 days) 36.

Viral Shedding and Infectiousness

Viral excretion data suggest that the average infectious period for influenza is about 1 day, with only a small percentage of cases (5%) remaining infectious for more than 2.9 days. This highlights that the window for effective isolation or treatment to prevent further transmission is quite short—ideally within 16 hours after symptoms start .

Transmission Routes and Environmental Survival

Influenza viruses are mainly spread through large respiratory droplets when an infected person coughs or sneezes, but can also be transmitted by touching contaminated surfaces and then touching the face. The virus can survive on hard, nonporous surfaces for 24–48 hours, and on hands for up to 5 minutes after contact with contaminated surfaces, making indirect transmission possible, especially in environments with heavy contamination 47.

Household Transmission Dynamics

Within households, the risk of infection for close contacts can be as high as 38%. The delay between the onset of symptoms in the first (index) case and secondary cases is typically around 3 days. Younger individuals are generally more susceptible to infection. Most secondary cases in households occur within 1–2 weeks of the index case becoming ill .

Summary of Key Transmission Periods

Incubation period: About 1–4 days (median 4 days for H7N9)
Infectious period (adults): 1 day before to 5–7 days after symptom onset
Infectious period (children): Can be longer than 7 days
Serial interval: 3–9 days
Peak transmission: 24–48 hours after infection begins
Environmental survival: Up to 48 hours on hard surfaces

Conclusion

The transmission period for influenza virus is relatively short but highly effective, with individuals able to spread the virus before and after symptoms appear. Most transmission occurs within the first few days of illness, and the virus can persist on surfaces long enough to facilitate indirect spread. Understanding these timelines is crucial for implementing timely isolation, treatment, and preventive measures to control influenza outbreaks 1234567.

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Most relevant research papers on this topic

Estimated Incubation Period and Serial Interval for Human-to-Human Influenza A(H7N9) Virus Transmission

The median incubation period for human-to-human transmitted avian influenza A(H7N9) virus infection is 4 days, with a serial interval of 9 days, suggesting a 10-day monitoring period for close contacts should detect most secondary infections.

Observational StudyRigorous Journal

2019·

8citations

·Lei Zhou et al.

Household Transmission of Influenza Virus

Household transmission studies provide valuable information on influenza transmission dynamics, with up to 38% of household contacts becoming infected after one infected member, and younger age being associated with higher susceptibility.

Literature ReviewHighly Cited

2015·

114citations

·T. Tsang et al.

EXPERIMENTAL TRANSMISSION OF INFLUENZA VIRUS INFECTION IN MICE

Infected mice can transmit influenza virus infection most readily 24 to 48 hours after infection, with a mouse-adapted A2 strain being more easily transmitted than a CAM strain of A1 virus.

1963·

53citations

·J. Schulman et al.

Influenza, Seasonal

Influenza spreads primarily through respiratory droplets, but also through direct contact, airborne aerosols, and small particle aerosols, with varying sensitivity and predictive values depending on community prevalence.

Highly Cited

2011·

742citations

Estimating influenza latency and infectious period durations using viral excretion data.

Influenza infection latency and infectious period durations are 1.6 and 1.0 days on average, with only 5% of cases infectious for more than 2.9 days, and isolation or treatment must be implemented shortly after symptoms onset to avoid 50% of secondary cases.

Highly Cited

2012·

71citations

·A. Cori et al.

Timelines of infection and transmission dynamics of H1N1pdm09 in swine

H1N1pdm09 influenza A virus in pigs shows a shorter infectious period (3.9 days) and a longer latent period (4.6 days) than previously estimated, aiding in understanding the spread of the virus in animal populations and potentially humans.

Non-RCTAnimal Study

2020·

17citations

·L. Canini et al.

Survival of influenza viruses on environmental surfaces.

Influenza viruses can survive on stainless steel surfaces for up to 8 hours and on paper tissues for up to 15 minutes, suggesting that transmission via fomites may be possible under heavy environmental contamination.

Highly Cited

1982·

550citations

·Bonnie Bean et al.

A discrete-time susceptible-infectious-recovered-susceptible model for the analysis of influenza data

The DT-SIRS model effectively captures seasonal influenza virus spread, with a 4-week period effect and variable transmission rates, compared to existing approaches.

2023·

7citations

·Georges Bucyibaruta et al.

Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature

Influenza virus transmission is influenced by both ambient relative humidity and temperature, with cold and dry conditions favoring transmission, and innate immune responses remain comparable between 5°C and 20°C temperatures.

Non-RCTAnimal StudyHighly Cited

2007·

1540citations

·Anice C. Lowen et al.

Viral Shedding and Transmission Potential of Asymptomatic and Paucisymptomatic Influenza Virus Infections in the Community

Asymptomatic and paucisymptomatic influenza cases have a higher potential for transmitting the virus to close contacts, despite shorter viral shedding durations.

Observational StudyRigorous JournalHighly Cited

2016·

167citations

·D. Ip et al.

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