BACKGROUND Predominance of 2 antigenically drifted influenza viruses during the 2019-2020 season offered an opportunity to assess vaccine effectiveness against life-threatening pediatric influenza disease from vaccine-mismatched viruses in the United States. METHODS We enrolled children aged <18 years admitted to the intensive care unit with acute respiratory infection across 17 hospitals. Respiratory specimens were tested using reverse-transcription polymerase chain reaction for influenza viruses and sequenced. Using a test-negative design, we estimated vaccine effectiveness comparing odds of vaccination in test-positive case patients vs test-negative controls, stratifying by age, virus type, and severity. Life-threating influenza included death or invasive mechanical ventilation, vasopressors, cardiopulmonary resuscitation, dialysis, or extracorporeal membrane oxygenation. RESULTS We enrolled 159 critically ill influenza case-patients (70% ≤8 years; 51% A/H1N1pdm09 and 25% B-Victoria viruses) and 132 controls (69% were aged ≤8 years). Among 56 sequenced A/H1N1pdm09 viruses, 29 (52%) were vaccine-mismatched (A/H1N1pdm09/5A+156K) and 23 (41%) were vaccine-matched (A/H1N1pdm09/5A+187A,189E). Among sequenced B-lineage viruses, majority (30 of 31) were vaccine-mismatched. Effectiveness against critical influenza was 63% (95% confidence interval [CI], 38% to 78%) and similar by age. Effectiveness was 75% (95% CI, 49% to 88%) against life-threatening influenza vs 57% (95% CI, 24% to 76%) against non-life-threating influenza. Effectiveness was 78% (95% CI, 41% to 92%) against matched A(H1N1)pdm09 viruses, 47% (95% CI, -21% to 77%) against mismatched A(H1N1)pdm09 viruses, and 75% (95% CI, 37% to 90%) against mismatched B-Victoria viruses. CONCLUSIONS During a season when vaccine-mismatched influenza viruses predominated, vaccination was associated with a reduced risk of critical and life-threatening influenza illness in children.

中文翻译：

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疫苗对美国儿童危及生命的流感疾病的有效性。

背景 2019-2020 流感季期间 2 种抗原性漂移流感病毒的优势提供了一个机会，可以评估美国针对疫苗不匹配病毒引起的危及生命的小儿流感疾病的疫苗有效性。方法 我们招募了 17 家医院因急性呼吸道感染入住重症监护病房的 18 岁以下儿童。使用流感病毒的逆转录聚合酶链反应测试呼吸道标本并测序。我们使用测试阴性设计，比较测试阳性病例与测试阴性对照的疫苗接种几率，按年龄、病毒类型和严重程度分层，估计了疫苗的有效性。威胁生命的流感包括死亡或有创机械通气、血管升压药、心肺复苏、透析、或体外膜氧合。结果 我们招募了 159 名重症流感病例患者（70% ≤ 8 岁；51% A/H1N1pdm09 和 25% B-Victoria 病毒）和 132 名对照（69% 年龄≤8 岁）。在 56 种已测序的 A/H1N1pdm09 病毒中，29 种 (52%) 与疫苗不匹配 (A/H1N1pdm09/5A+156K)，23 种 (41%) 与疫苗匹配 (A/H1N1pdm09/5A+187A,189E)。在测序的 B 系病毒中，大多数（31 种中的 30 种）疫苗不匹配。对严重流感的有效性为 63%（95% 置信区间 [CI]，38% 至 78%），并且年龄相似。对危及生命的流感的有效性为 75%（95% CI，49% 至 88%），而对非危及生命的流感的有效性为 57%（95% CI，24% 至 76%）。针对匹配的 A(H1N1)pdm09 病毒的有效性为 78%（95% CI，41% 至 92%），针对不匹配的 A(H1N1)pdm09 病毒的有效性为 47%（95% CI，-21% 至 77%），和 75%（95% CI，37% 至 90%）对抗不匹配的 B-Victoria 病毒。结论 在疫苗不匹配流感病毒占主导地位的季节，接种疫苗与儿童患严重和危及生命的流感疾病的风险降低有关。