Participants with documented prior A(H3N2) virus infection had higher pre-vaccine titres against strains circulating since 2004 compared to those without prior infection. Moreover, they had higher titre rises on days 7, 14, 21 and 280 post-vaccination against vaccine and subsequently circulating strains. Accordingly, 1/72 versus 4/28 of vaccinees with and without documented prior infection experienced illness due to A(H3N2) in the season after vaccination (p = 0.021). The range of A(H3N2) virus clades recognized by vaccine-induced antibodies was associated with the clade that last caused infection, indicating that recalled immunity drove antibody production against shared epitopes. InterpretationThese results suggest that immunological memory from prior infection drives and shapes antibody production induced by inactivated influenza vaccine, and underpins the capacity for vaccine to induce sufficient antibody for protection.

Rickettsial infections are recognized as important causes of fever throughout southeast Asia. Herein, we determined the seroprevalence to rickettsioses within rural and urban populations of northern Vietnam. Prevalence of individuals with evidence of prior rickettsial infections (IgG positive) was surprisingly low, with 9.14% (83/908) testing positive to the three major rickettsial serogroups thought to circulate in the region. Prevalence of typhus group rickettsiae (TG)–specific antibodies (6.5%, 58/908) was significantly greater than scrub typhus group orientiae (STG)– or spotted fever group rickettsiae (SFG)–specific antibodies (P < 0.05). The majority of TG seropositives were observed among urban rather than rural residents (P < 0.05). In contrast, overall antibody prevalence to STG and SFG were both very low (1.1%, 10/908 for STG; 1.7%, 15/908 for SFG), with no significant differences between rural and urban residents. These results provide data on baseline population characteristics that may help inform development of Rickettsia serological testing criteria in future clinical studies.

Rural farming communities in northern Vietnam do not routinely practice vaccination for influenza A viruses (IAV) for either humans or poultry, which enables us to study transmission intensity via seroepidemiology. Using samples from a longitudinal cohort of farming households, we determined the number of symptomatic and asymptomatic human infections for seasonal IAV and avian A/H9 over 2 years. As expected, we detected virologically confirmed acute cases of seasonal IAV in humans, as well as large numbers of subclinical seroconversions to A/H1pdm [55/265 (21 %)], A/H3 [95/265 (36 %)] and A/H9 [24/265 (9 %)]. Five of the A/H9 human seroconverters likely represented true infections rather than heterosubtypic immunity, because the individuals seroconverted solely to A/H9. Among co-located poultry, we found significantly higher seroprevalance for A/H5 compared to A/H9 in both chickens and ducks [for northern study sites overall, 337/1105 (30.5 %) seropositive for A/H5 and 123/1105 (11.1 %) seropositive for A/H9].

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