Pre-existing Antineuraminidase Antibodies Are Associated With Shortened Duration of Influenza A(H1N1)pdm Virus Shedding and Illness in Naturally Infected Adults

Abstract

Background: Influenza causes a substantial burden worldwide, and current seasonal influenza vaccine has suboptimal effectiveness. To develop better, more broadly protective vaccines, a more thorough understanding is needed of how antibodies that target the influenza virus surface antigens, hemagglutinin (HA) (including head and stalk regions) and neuraminidase (NA), impact influenza illness and virus transmission.

Methods: We used a case-ascertained, community-based study of household influenza virus transmission set in Managua, Nicaragua. Using data from 170 reverse transcriptase-polymerase chain reaction (RT-PCR)-confirmed influenza virus A(H1N1)pdm infections and 45 household members with serologically confirmed infection, we examined the association of pre-existing NA, hemagglutination inhibiting, and HA stalk antibody levels and influenza viral shedding and disease duration using accelerated failure time models.

Results: Among RT-PCR-confirmed infections in adults, pre-existing anti-NA antibody levels ≥40 were associated with a 69% (95% confidence interval [CI], 34-85%) shortened shedding duration (mean, 1.0 vs 3.2 days). Neuraminidase antibody levels ≥80 were associated with further shortened shedding and significantly shortened symptom duration (influenza-like illness, 82%; 95% CI, 39-95%). Among RT-PCR-confirmed infections in children, hemagglutination inhibition titers ≥1:20 were associated with a 32% (95% CI, 13-47%) shortened shedding duration (mean, 3.9 vs 6.0 days).

Conclusions: Our results suggest that anti-NA antibodies play a large role in reducing influenza illness duration in adults and may impact transmission, most clearly among adults. Neuraminidase should be considered as an additional target in next-generation influenza virus vaccine development.We found that antibodies against neuraminidase were associated with significantly shortened viral shedding, and among adults they were also associated with shortened symptom duration. These results support neuraminidase as a potential target of next-generation influenza virus vaccines.

Keywords: influenza; hemagglutination inhibition (HAI); household; neuraminidase; shedding.

Figure 1.

Pre-existing antibodies and influenza virus shedding duration among reverse transcriptase–polymerase chain reaction–confirmed infections. A–C, Histograms of hemagglutination inhibition, anti-hemagglutinin stalk, and anti-neuraminidase antibody levels by age. Histograms are colored according to influenza virus shedding duration. D–F, Event time ratios from accelerated failure time models, adjusted for age and sex, compare the shedding duration in those with high (threshold or higher) vs low (lower than threshold) antibody levels. An ETR <1 corresponds to a shorter duration in the high antibody group. Abbreviations: AFT, accelerated failure time; AUC, area under the curve; CI, confidence interval; ETR, event time ratio; HA, hemagglutinin; HAI, hemagglutination inhibition; NA, neuraminidase; RT-PCR, reverse transcriptase–polymerase chain reaction.

Figure 3.

Predicted influenza virus shedding duration by high and low pre-existing neuraminidase antibody levels. Curves show the proportion of people still positive by reverse transcriptase-polymerase chain reaction (y axis) each day since the start of shedding (x axis) for an adult female of average age (36 y). Mean (interquartile range) shedding times are presented in the inset. Abbreviations: IQR, interquartile range; NA, neuraminidase; Prop, proportion; RT-PCR, reverse transcriptase–polymerase chain reaction.

Figure 2.

Pre-existing antibody levels and influenza shedding duration among reverse transcriptase–polymerase chain reaction (RT-PCR)–and serologically confirmed infections. A–C, Histograms of hemagglutination inhibition, anti-hemagglutinin stalk, and anti-neuraminidase antibody levels by age. Histograms are colored according to influenza virus shedding duration. Shedding duration was set to 0.5 days for serologically confirmed, RT-PCR–negative individuals. D–F, event time ratios (ETRs) from accelerated failure time models, adjusted for age and sex, compare the shedding duration in those with high (threshold or higher) vs low (lower than threshold) antibody levels. An ETR <1 corresponds to a shorter duration in the high antibody group. Abbreviations: AFT, accelerated failure time; AUC, area under the curve; CI, confidence interval; ETR, event time ratio; HA, hemagglutinin; HAI, hemagglutination inhibition; NA, neuraminidase; RT-PCR, reverse transcriptase–polymerase chain reaction.

Figure 4.

Pre-existing neuraminidase (NA) antibodies and influenza virus shedding duration among reverse transcriptase–polymerase chain reaction–confirmed infections, adjusted for hemagglutination inhibition (HAI) and antihemagglutinin (HA) stalk antibodies. Event time ratios from accelerated failure time models compare the shedding duration in those with high (threshold or higher) vs low (lower than threshold) NA antibody levels. All models are adjusted for age and sex, and antibody-adjusted models are adjusted for HAI (titer ≥1:80) and anti-HA stalk antibodies (area under the curve ≥160). Abbreviations: AFT, accelerated failure time; AUC, area under the curve; CI, confidence interval; ETR, event time ratio; HA, hemagglutinin; HAI, hemagglutination inhibition; NA, neuraminidase; RT-PCR, reverse transcriptase–polymerase chain reaction.

Figure 5.

Pre-existing neuraminidase (NA) antibodies and symptom duration among reverse transcriptase–polymerase chain reaction–confirmed infections. A–C, Histograms of NA antibody levels by age. Histograms are colored according to duration of symptoms for influenza-like illness (ILI) (A), cough (B), and runny nose (C). Symptom duration was set to 0.5 days for individuals with no recorded symptoms. D–F, event time ratios (ETRs) from accelerated failure time models, adjusted for age and sex, compare the duration of ILI (D), cough (E), and runny nose (F) in those with high (threshold or higher) vs low (lower than threshold) antibody levels. An ETR <1 corresponds to a shorter duration in the high antibody group. Abbreviations: AFT, accelerated failure time; AUC, area under the curve; CI, confidence interval; ETR, event time ratio; ILI, influenza-like illness; NA, neuraminidase; RT-PCR, reverse transcriptase–polymerase chain reaction.