Seroprotection rates of vaccine-preventable diseases among newly arrived Eritrean asylum seekers in Switzerland: a cross-sectional study

Background: According to 2016 World Health Organization and United Nations Children's Fund country estimates, Eritrea has overall high vaccination coverage with immunization rates for three doses of diphtheria/tetanus/pertussis and polio vaccine of 95%, for two doses measles vaccine of 85% and for three doses hepatitis B vaccine of 85%. If confirmed, this could imply that routine basic vaccination of newly arrived Eritreans could be safely omitted. Methods: We used stored serum samples from two cross-sectional studies that screened newly arrived Eritrean refugees for infectious diseases. Consenting refugees aged 16 years and older who registered in one of three neighbouring cantons in northwestern Switzerland were enrolled between January 2016 and December 2017. Antibody titers against the following vaccine-preventable diseases were measured (applied thresholds for seroprotection in brackets): diphtheria (>0.1 IU/ml), tetanus (>0.1 IU/ml), measles (>150 mIU/ml), rubella (only for women, >11 IU/ml), varicella (>50 mIU/ml), hepatitis B [hepatitis B surface antigen (HBsAg) Index >0.9, Hepatitis B core antibody (anti-HBc) Index >0.9 and antibodies to HBsAg (anti-HBs) >10 IE/L]. Differences between sex and age groups (<= 25 and >25 years) were measured by Fisher's exact test. Results: We analysed samples of 133 study participants (20 women, 15%) with a median age of 25 years (range 16-61). Rates of seropositivity were as follows for women/men, respectively: diphtheria 57.9%/74.8% (difference non-significant), tetanus 94.8%/41.1% (P < 0.001), measles 73.7%/76.6% (non-significant), rubella in women 78.9%, varicella 89.5%/95.3% (non-significant), anti-HBc 15.8%/26.2% (non-significant) and anti-HBs 15.8%/17.8% (non-significant). Conclusion: Seroprevalence for vaccine-preventable infections did not meet levels required to confer herd immunity in any of the human-to-human transmissible diseases that were studied. In general, the strategy proposed by the Federal Office of Public Health to offer basic immunization to all newly arrived refugees, including newly arriving Eritrean refugees, is justified.