Haemophilus influenzae type b (Hib) causes pneumonia and meningitis almost exclusively in children under 5 years of age [1]. Before widespread vaccination in 2000, Hib was responsible for at least 8.13 million cases of serious disease in children (<5 years) and 371 000 deaths globally. Vaccination has dramatically reduced invasive Hib disease worldwide [2]. The pentavalent vaccine DTPw-HepB-Hib (Diphtheria-Tetanus-Pertussis (whole cell)-Hepatitis B-Haemophilus influenzae type b), containing purified capsular Hib polysaccharide (PRP) conjugated to the tetanus toxoid (carrier protein), was introduced in Lao PDR in 2009, replacing the DTPw-HepB vaccine. DTPw-HepB-Hib is scheduled at 6, 10 and 14 weeks of age [3]. In 2018, the coverage with the DTPw-HepB-Hib vaccine was 84% in Lao PDR [4]. Immunity against Hib can be determined by measuring antibody levels against the PRP immunogenic component of Hib conjugate vaccines [5].

There are little data on the level of immunity against Hib or the burden of disease in the general population in Lao PDR. Two serosurveys conducted in Bolikhamxay province found that 66.4% and 71.7% of children 8 to 28 months old had long-term Hib antibody protection (>1μg/ml) in 2013/14 and 2017 respectively [6].

To contribute to the understanding of the epidemiology of Hib in Lao PDR and the protection levels before and after the introduction of the vaccination, we tested serum samples from existing cohorts of vaccine age-eligible children and unvaccinated adolescents in the Lao PDR for antibodies against PRP. We also investigated possible predictors for long-term protection.

In this study, we found high long-term protection rates in adolescents born before the introduction of the vaccine. Almost all (95.9%) unvaccinated adolescents in cohort 1 showed evidence of natural exposure to H. influenzae type b, corresponding as expected to the high burden of Hib circulation in an unvaccinated population. There are no data at a population level documenting the Hib carriage or disease in Lao PDR. Our findings are similar to other serosurveys showing high seropositivity rates before the introduction of the vaccination in 2009. For example in Kathmandu, Nepal, 20% of under-fives had anti-Hib levels >0.15μg/ml, which increased to 83% among 15–54 year olds [20].

Even after the introduction of Hib vaccination, all of the unvaccinated children in cohort 3 (ARI contacts) were at least short-term protected and 25% showed evidence of long-term protection (Table 2). The reason for this is unknown but suggests on-going exposure/infections, incomplete documentation of vaccination status, or cross-reactive antibodies with Hib. Being recruited as ARI contacts, these children may be more likely to have been exposed to Hib. Nevertheless, the results from our unvaccinated children (cohort 1 and 3) suggest that Hib is still present in the community.

After the introduction of Hib vaccination, all children in cohort 3 and virtually all (99.5%) children in cohort 2 showed at least short-term protection. The proportion of unprotected children in cohort 2 (vaccinated children) was less than 1%. All children in cohort 2 and 52.3% of the children in cohort 3 received all three doses of DTPw-HepB-Hib vaccine; however, 41.1% in cohort 2 and 35.2% of the vaccinated children in cohort 3 had antibody titers insufficient for long-term protection. We reported low levels of antibodies against diphtheria, tetanus and hepatitis B, (components of the same pentavalent vaccine) without a clear explanation in cohort 2 previously [8]. A follow-up study in 2017 revealed that vaccine immunogenicity for diphtheria, tetanus and hepatitis B increased by around 20% compared to the study in 2013/14 [6]. However, the increase by 5% of long-term protection in response to the Hib component in 2017 was minimal (from 66.4% to 71.7%). Another reason for the large proportion of short-term protected children could be rapid waning of anti-Hib antibodies. Indeed, time since vaccination was associated with having a lower antibody titer in cohort 2 in the present study. Hib antibody waning has been reported previously and the introduction of a booster dose into the vaccination schedule has been discussed [2, 21, 22]. Furthermore, since data quality in the health sector has been of concern in Lao PDR before, we cannot exclude the possibility that the participants had not received the vaccination despite having documentation; or that they received the vaccination without having documentation [23].

None of the participant characteristics in cohort 1 was associated with long-term protection, indicating that at least in this setting, socio-economic factors did not seem to influence Hib infection. There were also no significant differences in long-term protection rates by location, ethnicity or most of the other factors within cohort 2 and 3, except for vaccination, time since vaccination and one of the nutritional parameters. The mid-upper arm circumference z-score < -2 was associated with shorter protection. Thus, malnutrition does not seem to have a major role in the response to Hib conjugate vaccine in this setting. A weak association between seroprotection against diphtheria and malnutrition was reported in cohort 2 before [8].

More data are needed to evaluate whether the vaccination led to a reduction in Hib infection in children. Other countries have reported a decrease of confirmed Hib related diseases after vaccine introduction using a primary three dose schedule as recommended by WHO [24–26]. A study in Kenya analysed 15 years of Hib surveillance data and found that the vaccine reduced the risk of Hib disease by 93% over this period. Additionally, they found that eight years after the introduction of the vaccine, 79% of children in the disease risk group, aged 4–35 months, had antibodies at levels indicating long-lasting protection [24]. In The Gambia, 13 years after the Hib vaccination was introduced as a primary 3-dose schedule, the vaccine remained highly effective in controlling invasive Hib disease [11]. Clinical data in addition to the monitoring of vaccine-induced protection rates could provide helpful information whether a booster is needed, using this study as baseline. Nevertheless, where Hib vaccine evaluations have been undertaken in low- and middle-income countries, a booster is not required to reduce Hib disease and therefore it is recommended that the Lao PDR schedule should not change at this time [24].

Limitations of our study may result from the different study designs of the original studies and laboratory methods. Participants in cohorts 2 and 3 were not selected randomly. Information regarding infection with H. influenza type b among the participants or other clinical information regarding the participants in the cohorts were not available. Furthermore, we assumed that participants born before vaccine introduction would be unvaccinated and there is no information regarding vaccination status of 34% of participants in cohort 3. This uncertainty may have affected the interpretation of our results. Lastly, our findings are from Central areas of the Lao PDR. Hib prevalence may vary between regions of Lao PDR and/or between different ethnic groups.

Our findings indicate that the circulation of Hib was high in Lao PDR before the introduction of the vaccine and continues to be high in unvaccinated children. Indeed, after vaccine introduction, all vaccinated children, but also all others showed serological markers of vaccination/past infection and protection. Thus, robust surveillance and systematic reporting of invasive Hib cases is required to determine the current burden of disease despite vaccination.