One US measles outbreak is proving particularly deadly. Here’s why

Hannah Beier/Reuters

An ongoing measles outbreak in Pennsylvania has quickly become the largest and deadliest in the United States in more than 30 years.

Historical data suggests that 1 to 3 people will die out of every 1,000 unvaccinated people who contract measles. But Pennsylvania has reported five measles-associated deaths — with a sixth under review — out of about 1,300 total reported cases since the start of the outbreak in April.

This puts the measles death rate in Pennsylvania at least 1.5 times higher than average — and possibly up to 5 times higher.

It’s not likely that the measles virus has changed. One factor might be the size of the outbreak and a probable undercount of cases: It’s difficult to know how many people have been affected. It may also be the unique vulnerabilities of the Pennsylvania community where measles is raging.

“There clearly is a shift — not in the biology but a shift in the epidemiology that we’re seeing here in Pennsylvania,” said Dr. Matthew Ferrari, director of the Center for Infectious Disease Dynamics at Penn State University.

Experts say the outbreak is even larger than official data suggests, and it’s not clear how big it is. The US successfully eliminated measles decades ago, making it difficult to estimate how bad the current resurgence has gotten.

A “disease detective” from the US Centers for Disease Control and Prevention arrived in Pennsylvania this week to help better understand the true scale of the outbreak and its trajectory, the state health department said. The scientist, with the CDC’s Epi-Aid program, is also tasked with evaluating the impact of local vaccination efforts.

Scientists and public health workers typically estimate current disease trends by using patterns from recent years, scaling changes against a few variables like birth rates and vaccination rates.

But measles data in the US is quite “outdated,” said Dr. Remy Pasco, a research associate at the University of Colorado Boulder. “It’s not endemic, so we don’t have as much data on it. And because the data has been so scarce, anything we can estimate is with a big grain of salt.”

Pasco has worked on an analysis to estimate the scale of last year’s measles outbreak in Texas. Some underreporting of cases is expected, he says, but it’s unlikely that the rate of underreporting would be the same — even when comparing recent outbreaks in different parts of the US.

There are some “peculiarities” with the outbreak in Pennsylvania, he said. Hospitalization rates with this outbreak are higher than expected. And the Amish and broader Plain community – so named for their simple way of dressing – has a unique way of interacting with the health care system.

Severe outcomes, such as hospitalizations and deaths, are generally the best benchmark for understanding how widespread a measles outbreak is, public health experts say. Not every person who is infected with measles will seek care or testing, but severe cases are more visible in the health care system, and tracking them can help set the ratio to understand the full scale.

One signal that this outbreak is different: A disproportionate share of cases are among adults.

About two-thirds of measles cases in Pennsylvania have been among adults, and all but a few were unvaccinated. Nationwide, however, cases skew younger; CDC data shows that more than 60% of cases in the US over the past two years have been among children or teens.

The large population of unvaccinated adults in Pennsylvania, particularly among Amish and other members of the Plain community, has created a uniquely risky outbreak situation, Ferrari said.

Measles complications tend to be more severe for unvaccinated adults than for unvaccinated children, he said. But the low rates of protection among adults in the community also mean vulnerable infants are more likely to be born without protective maternal antibodies.

“We normally think about measles as being a childhood infection,” Ferrari said. “Most people have been exposed to measles or have been vaccinated against measles by the time they reach reproductive age.”

That’s true around the world. Countries typically have high vaccination rates or endemic measles spread, which means people are exposed when they’re younger.

Much of the affected population in Pennsylvania falls into a uniquely susceptible group: both unvaccinated and going decades without exposure because the US had so few cases for so long.

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If there’s any explanation for a higher death rate than expected in Pennsylvania, Ferrari said, this would be it: Antibodies — whether from vaccination or infection — aren’t being passed on from women of reproductive age to their babies.

Two infants who were infected with measles have died in Pennsylvania this year, both too young to receive the measles-mumps-rubella (MMR) vaccine.

“Those are absolutely deaths that wouldn’t have been occurring elsewhere in the world,” Ferrari said.

The Pennsylvania Department of Health published an advisory Monday with guidance for measles in pregnancy and congenital measles.

“Due to the ongoing measles outbreak, there is an increase in measles cases during pregnancy, along with rising concerns about congenital measles, which has been identified for the first time in decades,” the advisory said.

Public health knowledge and guidance on this topic continues to develop, the health department said, but a measles infection increases the risk of complications for pregnant mothers and their babies.

Another complication to understanding this outbreak is an unusually political fight over measles deaths.

Pennsylvania leaders have clashed with federal leaders over measles deaths in Pennsylvania, putting a key data point for understanding the scale of the outbreak into question.

Although the state health department has officially reported five measles-associated deaths, the CDC dashboard includes only three — noting that the “number is subject to change” as additional information becomes available and that the agency is working to “develop a standardized case definition for deaths due to measles.”

In September, the Pennsylvania health department “began the formal process of requesting an Epi-Aid” from the CDC, but Dr. Debra Bogen, the department secretary, noted in a letter to CDC Director Dr. Erica Schwartz that the request would be rescinded “if CDC is unable to transparently and accurately communicate information about those measles-associated deaths.”

From Pennsylvania Department of Health/Facebook

Last week, Bogen sent another letter to Schwartz thanking her for the “support and technical assistance from CDC professional staff throughout this outbreak response” and accepting the Epi-Aid as the state health department and CDC staff “continue to collaborate effectively” on measles outbreak response work.

“While the Department is prepared to accept this Epi-Aid, I again ask the CDC to acknowledge the loss of five Pennsylvanians to measles and to report these deaths on the CDC’s public dashboard so that we can repair and rebuild trust in the public health system in the U.S.,” Bogen wrote. “By not acknowledging the deaths, the CDC obscures the seriousness of this disease and the toll of measles across the U.S.”

Without the historical data to use as a benchmark, understanding the true scale of the measles outbreak in Pennsylvania will require investigative public health work.

It’s “classic boots on the ground epidemiology,” Ferrari said, with epidemiologists out and about in the community asking questions about recent health history and vaccination coverage.

“These are outbreaks in populations that haven’t seen this in a long time, and so we can’t rely on the standard approaches that are predicated on consistent or persistent transmission.”

汉娜·拜尔/路透社

宾夕法尼亚州持续爆发的麻疹疫情已迅速成为美国30多年来规模最大、死亡人数最多的疫情。

历史数据显示，每1000名未接种疫苗的麻疹患者中，会有1至3人死亡。但自4月份疫情爆发以来，宾夕法尼亚州报告的麻疹相关死亡病例约1300例，其中5例死亡（另有1例正在调查中）。

这使得宾夕法尼亚州的麻疹死亡率至少比平均水平高出 1.5 倍，甚至可能高出 5 倍。

麻疹病毒不太可能发生变异。一个因素可能是疫情规模较大，导致病例数可能被低估：很难确定究竟有多少人受到影响。另一个因素可能是麻疹肆虐的宾夕法尼亚州社区的特殊脆弱性。

“很明显，宾夕法尼亚州正在发生一种转变——不是生物学上的转变，而是流行病学上的转变，”宾夕法尼亚州立大学传染病动力学中心主任马修·法拉利博士说。

专家表示，此次疫情的实际规模比官方数据显示的还要大，而且目前尚不清楚其具体规模。美国几十年前就已成功消灭了麻疹，因此很难估计此次疫情反弹的严重程度。

宾夕法尼亚州卫生部门表示，美国疾病控制与预防中心（CDC）的一名“疾病侦探”本周抵达宾州，以帮助更好地了解疫情的真实规模和发展趋势。这位来自CDC流行病援助项目（Epi-Aid）的科学家还负责评估当地疫苗接种工作的效果。

科学家和公共卫生工作者通常利用近年来的模式来估计当前的疾病趋势，并根据出生率和疫苗接种率等几个变量来调整变化。

科罗拉多大学博尔德分校的研究员雷米·帕斯科博士表示，美国的麻疹数据相当“过时”。“麻疹并非地方性流行病，所以我们掌握的数据并不多。而且由于数据如此匮乏，我们做出的任何估计都只能作为参考。”

帕斯科参与了一项分析，旨在估算去年德克萨斯州麻疹疫情的规模。他表示，病例漏报在所难免，但漏报率不太可能相同——即使比较美国不同地区近期爆发的疫情也是如此。

他说，宾夕法尼亚州的疫情有一些“特殊之处”。此次疫情的住院率高于预期。此外，阿米什人以及更广泛的平原社区（因其简朴的着装而得名）与医疗保健系统的互动方式也独具特色。

公共卫生专家表示，住院和死亡等严重后果通常是衡量麻疹疫情传播范围的最佳指标。并非所有麻疹感染者都会寻求治疗或检测，但重症病例在医疗系统中更容易被发现，追踪这些病例有助于确定相关比例，从而了解疫情的全貌。

此次疫情的一个不同之处在于：病例中成年人所占比例过高。

宾夕法尼亚州约三分之二的麻疹病例发生在成年人中，而且除少数人外，其余均未接种疫苗。然而，在全美范围内，病例的年龄分布更为年轻；美国疾病控制与预防中心的数据显示，过去两年美国超过60%的麻疹病例发生在儿童或青少年中。

法拉利表示，宾夕法尼亚州未接种疫苗的成年人口众多，尤其是阿米什人和其他平原社区成员，这造成了疫情爆发的特殊风险。

他表示，未接种疫苗的成年人患麻疹的并发症往往比未接种疫苗的儿童更为严重。但社区成年人群体中较低的疫苗保护率也意味着，易感婴儿更有可能出生时缺乏母体保护性抗体。

“我们通常认为麻疹是一种儿童传染病，”法拉利说。“大多数人在达到生育年龄之前都接触过麻疹病毒或接种过麻疹疫苗。”

这种情况在世界各地都存在。这些国家通常疫苗接种率高或麻疹流行，这意味着人们在年幼时就可能接触到麻疹病毒。

宾夕法尼亚州受影响的大部分人口属于一个特别易感的群体：既没有接种疫苗，又因为美国长期以来病例很少而几十年都没有接触过病毒。

CNN记者米歇尔·古斯塔夫森报道

在数十年来最严重的麻疹疫情爆发之际，一家诊所依靠多年来建立的来之不易的信任，保障弱势群体的安全。

法拉利表示，如果宾夕法尼亚州的死亡率高于预期有什么解释的话，那就是：无论是接种疫苗还是感染产生的抗体，都没有从育龄妇女传递给她们的婴儿。

今年宾夕法尼亚州有两名感染麻疹的婴儿死亡，他们都太小，无法接种麻疹-腮腺炎-风疹 (MMR) 疫苗。

“这些死亡事件在世界其他地方绝对不会发生，”法拉利说。

宾夕法尼亚州卫生部周一发布了一份咨询报告，就孕期麻疹和先天性麻疹提供了指导。

“由于麻疹疫情持续爆发，孕期麻疹病例有所增加，同时，人们对先天性麻疹的担忧也在加剧，这是几十年来首次发现先天性麻疹病例，”该公告称。

卫生部门表示，关于麻疹的公共卫生知识和指导仍在不断发展，但麻疹感染会增加孕妇及其婴儿出现并发症的风险。

了解此次疫情的另一个复杂之处在于，围绕麻疹死亡人数的争论异常激烈，甚至演变成一场政治斗争。

宾夕法尼亚州领导人与联邦领导人就宾夕法尼亚州的麻疹死亡人数发生冲突，这使得了解疫情规模的关键数据点受到质疑。

尽管州卫生部门已正式报告了 5 例与麻疹相关的死亡病例，但美国疾病控制与预防中心 (CDC) 的仪表盘上仅包含 3 例——并指出随着更多信息的出现，“数字可能会发生变化”，而且该机构正在努力“制定麻疹死亡病例的标准化定义”。

9 月，宾夕法尼亚州卫生部门“正式启动了向美国疾病控制与预防中心 (CDC) 申请流行病援助的程序”，但该部门秘书黛布拉·博根博士在给 CDC 主任埃里卡·施瓦茨博士的一封信中指出，如果“CDC 无法透明、准确地传达有关麻疹相关死亡的信息”，则该请求将被撤销。

来自宾夕法尼亚州卫生部/Facebook

上周，博根再次致信施瓦茨，感谢她“在此次疫情应对过程中，疾控中心专业人员提供的支持和技术援助”，并接受了流行病援助，因为州卫生部门和疾控中心工作人员“将继续有效合作”开展麻疹疫情应对工作。

“虽然卫生部准备接受这份流行病援助，但我再次要求疾控中心承认五名宾夕法尼亚州居民死于麻疹，并在疾控中心的公共信息平台上公布这些死亡病例，以便我们能够修复和重建公众对美国公共卫生系统的信任，”博根写道。“疾控中心不承认这些死亡病例，掩盖了这种疾病的严重性以及麻疹在美国造成的惨重损失。”

由于缺乏历史数据作为基准，要了解宾夕法尼亚州麻疹疫情的真实规模，需要开展公共卫生调查工作。

法拉利说，这是“经典的实地流行病学调查”，流行病学家们走入社区，询问居民最近的健康史和疫苗接种覆盖率。

“这些疫情爆发的地区很久没有出现过这种情况了，因此我们不能依赖基于持续或稳定传播的标准方法。”