Studies show science behind CDC pandemic response

Dec 14, 2010 (CIDRAP News) – The US Centers for Disease Control and Prevention (CDC) today published a detailed look at the early field and epidemiologic studies it used to guide its response to the 2009 H1N1 pandemic, such as spread among household contacts, disease severity, and the impact on high-risk groups.

The 29 reports, all of which are freely available, appear today in an early online supplement of Clinical Infectious Diseases (CID) that is sponsored by the CDC. Topics range from epidemiologic and surveillance methods used by the CDC to clinical characteristics and disease burden.

In introductions to the supplement series, some of the CDC officials who played key leadership roles in the pandemic response described the challenges they faced when the novel H1N1 virus was first detected in two California children in April 2009.

Unlike the past two pandemics, when the new viruses emerged in Asia, CDC officials said they didn't have any advance warning. The rapid spread of the virus at the US-Mexico border meant the agency had to devise ways to quickly evaluate and respond to the threat.

Based on its early investigations, the CDC was able to piece together the outbreak profile of the new virus fairly early: a rapidly spreading virus of mild-to-moderate severity that hit children and young adults the hardest, along with other risk groups such as morbidly obese people and minority populations such as Native Americans.

Comparing 1976 and 2009
The package of reports leads with a perspective article on the 2009 H1N1 pandemic from Dr David Sencer, who was director of the CDC during the 1976 swine-origin H1N1 outbreak and vaccination campaign. Sencer also served as an advisor to the CDC during the 2009 H1N1 pandemic.

Sencer described several differences between the two outbreaks and speculated that lessons learned during the 1976 outbreak and other emergency response efforts helped shape the CDC's response to the 2009 H1N1 pandemic. He noted that early events in 1976, such as controversy over vaccine maker indemnification and the media's mischaracterization of the cost of the vaccine, jaded the public's perception of the 1976 swine flu vaccine.

Events surrounding the 1976 vaccine seemed politicized and without much transparency, according to Sencer, but he wrote that during the 2009 pandemic the CDC took on a highly visible role as the lead agency and helped build scientific credibility through consistent messaging that clearly acknowledged uncertainty.

Though surveillance systems in 2009 featured more advanced technology, the nation still lacks a national uniform automated system that can provide real-time data and better information about, for example, vaccine updates, he observed.

Alluding to the early administration of the 1976 pandemic vaccine, Sencer emphasized that making decisions during public health crises. He said he still believes the rapid rollout of the 1976 vaccine was the right approach, but events surrounding the vaccine campaign became more damaging when health officials did little to contest the barrage of media criticism.

He wrote that the CDC fared much better during the 2009 pandemic, because it engaged the media early on with "boot camps" and inclusion in pandemic exercises. "The CDC has handled the response to the pandemic with great scientific skill and has navigated the shoals of the bureaucracy adroitly," Sencer wrote.

"Perhaps the primary lesson learned from this pandemic will be that while decision-making is always risky, that risk can be minimized through effective communications," he added.

Gauging disease burden and patterns
In a detailed report on the overall epidemiology of the 2009 H1N1 pandemic, CDC experts highlighted the usefulness of providing periodic estimates of illnesses, hospitalizations, and deaths, reports that are usually issued well after a typical flu season has ended.

However, the greater capacity for lab confirmation of pandemic flu and improved surveillance helped the CDC issue more frequent updates, which helped it understand and communicate severity.

The emergence of the triple-reassortant H1N1virus served not only as a reminder of how unpredictable influenza can be, the CDC authors wrote that it also reinforces the importance of including the lack of population immunity to novel strains into pandemic planning. It also underscored the need to address gaps in flu surveillance in domestic animals.

CDC field investigators were exploring hospitalization and death patterns over the course of the pandemic, with an eye toward picking up any possible differences between the two pandemic waves. For example, a clinical case series of 225 patients hospitalized during the fall wave showed that clinical severity did not change substantially from the spring to fall waves.

A study to gauge patterns with deaths during the first pandemic wave, based on 377 reports, told CDC officials that mortality rates were low, but most deaths were occurring in people younger than age 65 who had underlying medical conditions. The findings led to stronger recommendations that children and adults with chronic conditions be treated early and more aggressively with antiviral medications.

When they looked more closely at 2009 H1N1 deaths in children compared with seasonal flu, researchers found that children who were dying from the novel virus were slightly older and had underlying medical conditions, which the CDC said highlighted the importance of vaccination for all children, particularly those at higher risk for flu complications.

Finding transmission clues
Given the severity pattern that was emerging with the new virus, the CDC learned that to better estimate disease incidence it needed to identify the number of sick patients who didn't seek medical care, a group likely to fall through the cracks of surveillance systems.

Using a 10-state telephone survey, scientists estimated that community incidence early in the outbreak was 4.7 per 100 adults.

During the pandemic the CDC used household surveys to help flesh out transmission patterns and guide its school closure advice, and a study among children early in the outbreak suggested that viral shedding was similar to seasonal flu, which helped inform the CDC's recommendations on returning to school and work.

During the first reported outbreak in a university setting, CDC researchers launched an online student survey, which confirmed that close contact or prolonged exposure to sick people was linked to influenza-like illness, findings that guided mitigation efforts in university populations.

Another survey of nonpharmacologic interventions at a college campus showed that although a high number of students and faculty adhered to mitigation advice, only small percentages stayed home when they were sick. These data demonstrated a need for boosting compliance with social-distancing measures.

Some of the CDC's field investigations also showed a need to better evaluate the effectiveness of nonpharmaceutical interventions in households and the need to determine the effectiveness of school closures during outbreaks (studies on an elementary school closure, lessons learned from closures, and the economic impact of closures).

Tracking impact on risk groups
Early in the outbreak, health experts raised concerns about the possible impact of the pandemic on people with impaired immune systems, those in certain ethnic groups, and healthcare workers who were more likely to be in contact with sick patients.

A study detailing three case series found that among people hospitalized with 2009 H1N1 infections, those who also had HIV infections had similar rates of intensive care unit admissions and deaths as patients who didn't have HIV infections. However, they found that only about a third of HIV patients with pandemic flu received antiviral medications within 48 hours of illness onset, which pointed to the need for earlier empiric treatment in this population.

Public health officials raised concerns about the potential impact of the new virus on Alaska's native people, who were hit hard during the 1918 pandemic. A CDC analysis of syndromic data revealed that a pandemic wave occurred in Alaska in August and September of 2009, with disease occurring first in rural regions where the state's native people live. The study also found that Alaska natives and Pacific Islander natives were two to four times more likely than white people to be hospitalized with pandemic H1N1 infections.

The CDC concluded that the findings in native people have implications for the distributing of antivirals and vaccines in pandemic settings.

In looking at disease patterns in healthcare workers, the CDC analyzed illness reports for 70 workers from 22 different states. About half were infected in healthcare settings, and the CDC found that exposure to other sick workers likely played a role in some illnesses. In addition, some sick workers reported not wearing an N-95 respirator or surgical mask during all contact with sick flu patients.

The patterns revealed that comprehensive flu infection control strategies are needed in healthcare settings, including vaccination, managing sick workers, and adhering to respiratory protection and other infection control guidance.

See also:

Jan 1, 2011, CID supplement

Dec 14 CDC press release

Newsletter Sign-up

Get CIDRAP news and other free newsletters.

Sign up now»


Unrestricted financial support provided by

Bentson Foundation 3MAccelerate DiagnosticsGilead 
Grant support for ASP provided by


  Become an underwriter»