| Frank (35) |
Surveillance in families, Houston, Texas, 1975–1979 |
In a study of 50 influenza A illnesses in 41 children <4 years of age, presymptomatic viral shedding was noted in 4 (8%) from whom positive samples were obtained 6, 4, 3, and 3 days, respectively, before symptom onset. Samples were not cultured for most children before symptom onset. Peak shedding occurred during week 1 of illness, when 73% of cultures were positive; 10% were positive on days 8–11, 5% on days 12–15, and 0% days 16–19. Illness duration and shedding were not correlated. |
In children, at least 8% of illnesses in a seasonal outbreak were associated with presymptomatic shedding, which was noted up to 6 days before illness; 5% continued to shed at the end of week 2. Peak rates of shedding correlated with symptoms. |
| Hall (36) |
Children with respiratory illness seeking medical care during influenza outbreaks, Rochester, New York, 1977 and 1986 |
In an influenza B study of 43 children (mean age 8 y), 74% had typical influenzalike illness, 7% had afebrile upper respiratory infections, and 19% had croup. Peak virus titers occurred on day 1 of illness, and 93% of children shed virus on days 1–3, 75% on day 4, and 30% on day 6. Severe illness and prolonged fever were associated with higher virus titers, and a trend (p<0.07) for younger children to shed higher-titer virus occurred. In an influenza A study, symptoms and the highest viral shedding occurred early in illness; 30% of patients still shed at day 7. |
Peak shedding early in illness, correlated with symptoms and fever. |
| Davis (37) |
Surveillance in families, Washington, DC area, 1951–1956 |
Influenza A virus was recovered from 0/4 and 2/3 throat swab samples obtained 3–5 and 1–2 days, respectively, before symptoms, compared with 75 (59%) of 127 swab specimens collected within 3 days after onset of illness. |
Shedding at low levels occurred 1–2 days before symptoms |
| Foy (38) |
Surveillance in families, Seattle, Washington, 1984 |
12/37 persons, all >12 y of age, from whom influenza B virus was isolated were asymptomatic. Virus cultures from asymptomatic persons required longer incubation time than those from symptomatic persons to become positive (12.7 vs. 8.7 days, p<0.007), suggesting low titers of virus in these specimens. The study design did not permit assessment of whether asymptomatic persons transmitted influenza. |
Asymptomatic shedding at a low titer may be common during a seasonal outbreak. |
| Murphy (39) |
In the control arm of a study, 7 adult volunteers were nasally infected with wild-type H3N2 virus |
Peak virus shedding occurred 2 days after nasal inoculation, and shedding ceased by day 6. Titer of virus shed and daily fever score (reflects height and duration of fever) were strongly correlated (p<0.001). 1/7 study participants had mild symptoms without fever and shed less virus. |
Fever score and quantity of virus shed were strongly correlated. |
| Couch (40) |
In a vaccine study, 29 adults received vaccines, and 11 controls received saline. All were nasally infected with H3N2 |
The vaccinated group had a reduced rate and severity of illness. Among controls (without specific antibody), titer of virus shed and daily severity of illness scores (p<0.001) were strongly correlated. In the combined group, peak viral shedding occurred 3 days after infection. Virus was shed a mean of 5.7 days. 6/29 vaccine recipients and 5/11 controls shed low levels of virus but did not become ill. |
Severity of illness and quantity of virus shed were strongly correlated; a substantial rate of asymptomatic shedding occurred at low titer. |
| Khakpour (41) |
Daily cultures of 29 adult prisoners after natural exposure to influenza, Iran, 1968 |
Virus was isolated from 5/29 contacts. One person had influenza isolated from a throat washing and blood, which is rare, 12 h before symptoms. Four persons had virus isolated from throat washings but never had symptoms; acute antibody titer was high in 2. Virus titrations were not performed. |
Presymptomatic and asymptomatic shedding can occur. |
| Philip (42) |
Surveillance in families, Washington, DC area, 1952–1955 |
In households with proven influenza illness, influenza A was isolated from 8 (38%) of 21 contacts with acute afebrile respiratory illness and 2 (5%) of 40 healthy contacts. Influenza B virus was isolated from 2 (11%) of 19 contacts with acute afebrile respiratory illness and 0 of 47 contacts. Age distribution of culture-positive contacts was not reported. |
Symptomatic household contacts commonly shed influenza A virus. Asymptomatic shedding of influenza A and B viruses was uncommon. |
| Monto (43) |
Surveillance in families, Tecumseh, Michigan, 1976–1981 |
During seasonal epidemics among persons with serologically proven influenza A (H3N2) infection, at least 15%–25% were ill. Among persons with serologically proven influenza B, at least 19%–34% were ill. Among persons with febrile respiratory illness, influenza virus was isolated from 19.7%. |
≈60% of seasonal infections were asymptomatic. (This includes persons with partial immunity due to infection with the same subtype as in previous years; the percentage of asymptomatic infections would be lower in a pandemic.) Viral shedding in asymptomatic persons is likely to be quite low. |
| Hayden (44) |
19 volunteers infected with influenza A H1N1 |
All study participants were symptomatic, and symptom scores peaked on day 2 and returned to normal by day 8. Virus titers correlated with symptoms. |
Viral shedding was correlated with symptoms. |
