Antibody levels in a cohort of pregnant women after the 2009 influenza A(H1N1) pandemic: Waning and association with self‐reported severity and duration of illness

Abstract

Background A population‐based pregnancy cohort was established in Norway to study potential effects of exposure to the 2009 influenza pandemic or pandemic vaccination during pregnancy. Objectives We studied maternal A(H1N1)pdm09‐specific hemagglutination inhibition ( HI )‐titer levels and waning in women with influenza‐like illness ( ILI ) in pregnancy compared to vaccinated women. Moreover, we studied the association between HI ‐titers and self‐reported severity and duration of ILI . Methods HI ‐titers against the pandemic virus were measured in maternal blood samples obtained at birth, 3‐9 months after exposure, and linked with information about pregnancy, influenza and vaccination from national registries and a cohort questionnaire. Results Among 1821 pregnant women included, 43.7% were unvaccinated and 19.3% of these had ILI . HI ‐titers were low (geometric mean titer ( GMT ) 11.3) in the unvaccinated women with ILI . Higher HI ‐titers ( GMT 37.8) were measured in the vaccinated women. Estimated HI ‐titer waning was similar for vaccinated women and women with ILI . Most ILI episodes were moderate and lasted 3‐5 days. Women with ILI reporting specific influenza symptoms such as fever or cough had higher HI ‐titers than women without these symptoms. Women who reported being “very ill” or illness duration of >5 days had higher HI ‐titers than women reporting less severe illness or illness of shorter duration, respectively. Conclusions Antibody waning was similar in vaccinated women and women with ILI . More severe ILI or longer duration of illness was associated with higher HI ‐titers. Most unvaccinated pregnant women with ILI had low HI ‐titers, probably due to moderate illness and HI ‐titer waning between exposure and sampling.

Detection of anti‐NS1 antibodies after pandemic influenza exposure: Evaluation of a serological method for distinguishing H1N1pdm09 infected from vaccinated cases

“…The main limitation of this study was the considerable time between human pandemic infection (>6 months) and sampling, during which antibody titres against NS1 and HA are likely to have waned, 17 and in some samples probably below the limit of detection. This is in contrast to the control animal sera, which were collected 2-4 weeks post-infection.…”

“…8 The main strength of this study was access to human samples from a well-characterized cohort, combining national registry data, self-reported questionnaire data and HI titre data. 17,21,22 During the pandemic, it was mandatory to report LCI and vaccination to the The "+" and "−" signs reflect the case definitions as follows: Uninfected, unvaccinated controls: "−/−," LCI, unvaccinated cases: "+/−," and Vaccinated cases without ILI: "−/+". misclassification of vaccination status is unlikely, 18 as supported by the corresponding HI titre data.…”

“…However, previous analysis on a larger sample size from the same pregnancy cohort supported a correlation between HI titre and disease severity. 17 Another limitation was the large degree of variation in LU values between measurements, suggesting the LIPS method is more appropriate for defining serostatus than antibody quantification, at least for H1N1pdm09. It was not sensitive enough for screening the remaining participants for H1N1pdm09 infection in our pregnancy cohort where LCI data are lacking.…”

“…Human plasma samples were obtained from the Norwegian Influenza Pregnancy Cohort Study (NorFlu) established during the A/H1N1 pandemic in 2009, as described previously . The study was approved by the Regional Committees for Medical and Health Research Ethics South East (2009/2165).…”

“…H1N1pdm09, influenza, NS1, pandemic, serology, vaccination previously. 17 If there was no HI at the highest serum concentration (1:10 dilution), the titre was assigned a value of 5 for calculation purposes.…”