Results
For 39 experimental and 83 naturally infected influenza A H3 cases,
there were 84 and 146 exhaled breath collection instances respectively.
Of the 39 confirmed experimental cases, 36 were qRT-PCR positive two or
more days, 31 of whom also had serological evidence of infection; three
were qRT-PCR positive on one day only and had serological evidence of
infection. A total of 52 challenge study volunteers were inoculated,
giving an infection rate of 75%, based on the current case infection
definition.
Both study populations of young adults were generally healthy. The
experimental group was on average 10 years older than the naturally
infected. Experimental cases were more likely to be male while naturally
infected cases were balanced by sex (Table 1). Experimental cases had
illness mostly characterized by upper respiratory symptoms, or were
asymptomatic (12.8%) (Table 1). There were small peaks in upper
respiratory, lower respiratory, systemic, and cough scores, and cough
counts on day 3 post inoculation. Naturally infected cases had more
severe symptoms scores and greater cough counts, with symptom scores
peaking on day 1 post symptom onset and aligning with day 3 post
inoculation (Figure 1).
The risk of shedding virus into coarse and fine aerosols for
experimentally infected was 6/39 (15%) and 11/39 (28%), and for
naturally infected 45/83 (54%) and 71/83 (86%) (Table 2, Figure S4a).
Median coarse and fine aerosol shedding quantity between the groups was
significantly higher for natural cases in both fine (p<0.001).
and coarse (p<0.001) aerosol fractions. Peak aerosol shedding
was observed on day 3 post inoculation and day 1 post symptom onset for
experimental and natural cases, respectively, matching peak symptom
score day alignment (Figure 2). No virus was detected in fine aerosols
on day 1 post inoculation in experimental cases. On the day of peak
aerosol shedding, median symptom scores for experimental infection were
upper respiratory 4 (IQR 2, 5), lower respiratory 0 (0, 1), systemic 1
(0, 2), cough 0 (0, 1), and cough count 0 (0, 6) and for natural
infections 7 (5, 9), 3 (2, 4), 6 (4, 8), 2 (2, 3), and 22 (8, 40),
respectively.
When using all the detectable qRT-PCR nasopharyngeal swab samples from
days 1-6 post inoculation (N=179) and 1-3 post symptom onset (N=143), Ct
values were notably lower for natural compared with experimental cases
(Figure S2). The lowest Ct values were seen on day-1 post symptom onset
in naturally infected cases and day-3 post inoculation; subsequent days
showed a faster rise in Ct for experimental compared with natural cases.
Restricted to maximum shedding observations by aerosol fraction, the GM
(GSD) for coarse and fine aerosols was 2.7E+3 (3.3) and 5.1E+3 (4.7) for
experimentally, and 2.1E+4 (16.5) and 5.1E+4 (17.0) for naturally
infected cases (Table 2, Figure S3). Descriptive statistics for
covariates during maximum fine aerosol shedding observation days (Table
S3) are similar to those derived from all observation days. Upper
respiratory symptoms score distributions overlapped the most between
groups, while differences in the distributions of lower respiratory,
systemic, and cough symptoms scores, and cough count were more
pronounced (Figure 3).
For the experimental cases, unadjusted upper and lower respiratory
scores, cough symptoms, and cough count were positively associated with
fine aerosol shedding detection (Table 3). For naturally infected cases,
unadjusted lower respiratory symptom scores, and cough symptoms were
positively associated with fine aerosol shedding risk. All symptom
related covariates were associated with aerosol shedding risk after
combining both populations. Study day was negatively associated with
aerosol shedding in naturally infected cases. Nasopharyngeal swab
qRT-PCR cycle threshold (Ct) value had a negative association with
aerosol shedding risk for both groups, separately and combined. There
were no significant predictors of aerosol shedding rate among
experimental or natural cases, and only after combing groups did body
temperature reach a significant association. Of the experimentally
infected, only males shed detectable virus into aerosols.
After extensive testing, the best propensity score model (covariates:
fever >37.8oC, body temperature, and
upper respiratory symptom score) and adjustment by inverse probability
weighting for average treatment effect (ATE) minimized the standardized
differences between the groups with mean absolute value standardized
difference of 91 (Table S5). After ATE adjustment, balance improved for
some covariates, however not to the point where they could be considered
similar (Figure 4, Table S5). Although it is advisable that absolute
standardized differences for covariates not exceed 10% between
comparison groups, and variance ratios approach one (range
0.5-2),18 the best model with weighted adjustment had
absolute standardized differences ranging 7.3% to 169.1% and variance
ratios up to 48.5. The substantial differences between covariate
distributions did not support the use of propensity score adjusted
approaches for making further comparisons between these populations.