Introduction: Influenza A(H3N2) clades 3C.2a and 3C.3a co-circulated in Europe in 2018/19. Immunological imprinting by first childhood influenza infection may induce future birth cohort differences in vaccine effectiveness (VE). Aim: The I-MOVE multicentre primary care test-negative study assessed 2018/19 influenza A(H3N2) VE by age and genetic subgroups to explore VE by birth cohort. Methods: We measured VE against influenza A(H3N2) and (sub)clades. We stratified VE by usual age groups (0-14, 15-64, ≥ 65-years). To assess the imprint-regulated effect of vaccine (I-REV) hypothesis, we further stratified the middle-aged group, notably including 32-54-year-olds (1964-86) sharing potential childhood imprinting to serine at haemagglutinin position 159. Results: Influenza A(H3N2) VE among all ages was −1% (95% confidence interval (CI): −24 to 18) and 46% (95% CI: 8-68), −26% (95% CI: −66 to 4) and 20% (95% CI: −20 to 46) among 0-14, 15-64 and ≥ 65-year-olds, respectively. Among 15-64-year-olds, VE against clades 3C.2a1b and 3C.3a was 15% (95% CI: −34 to 50) and −74% (95% CI: −259 to 16), respectively. VE was −18% (95% CI: −140 to 41), −53% (95% CI: −131 to −2) and −12% (95% CI: −74 to 28) among 15-31-year-olds (1987-2003), 32-54-yearolds (1964-86) and 55-64-year-olds (1954-63), respectively. Discussion: The lowest 2018/19 influenza A(H3N2) VE was against clade 3C.3a and among those born 1964-86, corresponding to the I-REV hypothesis. The low influenza A(H3N2) VE in 15-64-year-olds and the public health impact of the I-REV hypothesis warrant further study.

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Over the last three years an unprecedented flow of migrants arrived in Europe. There is evidence that vaccine preventable diseases have caused outbreaks in migrant holding centres. These outbreaks can be favored by a combination of factors including low immunization coverage, bad conditions that migrants face during their exhausting journey and overcrowding within holding facilities. In 2017, we conducted an online survey in Croatia, Greece, Italy, Malta, Portugal and Slovenia to explore the national immunization strategies targeting irregular migrants, refugees and asylum seekers. All countries stated that a national regulation supporting vaccination offer to migrants is available. Croatia, Italy, Portugal and Slovenia offer to migrant children and adolescents all vaccinations included in the National Immunization Plan; Greece and Malta offer only certain vaccinations, including those against diphtheria-tetanus-pertussis, poliomyelitis and measles-mumps-rubella. Croatia, Italy, Malta and Portugal also extend the vaccination offer to adults. All countries deliver vaccinations in holding centres and/or community health services, no one delivers vaccinations at entry site. Operating procedures that guarantee the migrants' access to vaccination at the community level are available only in Portugal. Data on administered vaccines is available at the national level in four countries: individual data in Malta and Croatia, aggregated data in Greece and Portugal. Data on vaccination uptake among migrants is available at national level only in Malta. Concluding, although diversified, strategies for migrant vaccination are in place in all the surveyed countries and generally in line with WHO and ECDC indications. Development of procedures to keep track of migrants' immunization data across countries, development of strategies to facilitate and monitor migrants' access to vaccinations at the community level and collection of data on vaccination uptake among migrants should be promoted to meet existing gaps.

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