Estimates of the Prevalence of Pandemic H1N1(2009)-United States

[indigo girl]

Estimates of the Prevalence of Pandemic (H1N1) 2009, United States,

April–July 2009

http://www.cdc.gov/eid/content/15/12/pdfs/09-1413.pdf

This study will be released in December and only covers the first four months of the pandemic in the US. These are only estimates, and the full study is available at the link above.

Conclusions

We demonstrate that the reported cases of laboratory confirmed pandemic (H1N1) 2009 are likely a substantial underestimation of the total number of actual illnesses that occurred in the community during the spring of 2009. We estimate that through July 23, 2009, from 1.8 million to 5.7 million symptomatic cases of pandemic (H1N1) 2009 occurred in the United States, resulting in 9,000–21,000 hospitalizations. We did not estimate the number of deaths directly from our model, but among reports of laboratory-confirmed cases though July 23, the ratio of deaths to hospitalizations was 6%. When applying this fraction to the number of hospitalizations calculated from the model—that is, by assuming that deaths and hospitalizations are underreported to the same extent—we obtain a median estimate of 800 deaths (90% range 550– 1,300) during this same period. Because this assumption has several limitations (5), more sophisticated models are also being developed to better understand the severity of the US epidemic in the spring of 2009, including intensive care unit admissions and deaths (6).

Our analysis involves several assumptions. Data for parameter estimates were collected in limited periods and areas and thus may not be fully representative of the entire United States. To account for some of this uncertainty, a range of values was included for each proportion. Additional data from surveys of health-seeking behavior, physician testing practices, and policies for confirmatory testing at public health laboratories could help refine the parameter estimates. In addition, parameters were obtained from studies of persons with ILI, defined as fever with cough or sore throat. Persons with milder illness may be less likely to seek care or be tested, and thus may not be fully captured in these estimates. Likewise, in some heavily affected areas, the size of the outbreak quickly exceeded the capacity to ascertain and test case-patients. Thus, our results may reflect a conservative estimate of total cases.

As pandemic (H1N1) 2009 continues to spread through the United States and the world, laboratory-confirmed cases will continue to greatly underestimate the number of actual cases that occur. Surveillance for influenza does not traditionally rely on complete case ascertainment, which would be impractical, but on focused case ascertainment with well-characterized surveillance systems and special studies. Unfortunately, relying on laboratory-confirmed cases limits the ability to understand the full impact and severity of the epidemic, especially when severe cases are more likely to be recognized (5).

This model provides a relatively quick and simple approach to estimate the human health impact of the epidemic in advance of more rigorous analysis of surveillance and health care data that will be available over the next few years. Health systems and infrastructure may be unprepared in the short-term if plans are based on a number of confirmed cases that substantially

underestimates the impact of the epidemic. We estimate that the total number of pandemic (H1N1) 2009 cases in the United States during April–July 2009 may have been up to 140× greater than the reported number of laboratory confirmed cases. A spreadsheet version of the model has been developed and is available online (http://www.cdc.gov/h1n1flu/tools). Using this tool, health officials and policy makers could adjust the model parameters to represent their local experience, which may provide useful estimates of the prevalence of pandemic (H1N1) 2009 in their areas and help plan for a subsequent wave of the epidemic in the fall and winter months of 2009–2010.

(hat tip flutrackers/Laidback Al)

[lamazeteacher]

Your vigilance and dedication to reporting the spread and mortality this pandemic costs, is admirable and brave. Through these threads you created, I'm sure many lives will have been saved through vaccination or early action if/when the infection occurs.

Thank you so much.

Lois

[indigo girl]

You are very welcome, lamaze.

The link below should accompany the study.

http://www.cdc.gov/h1n1flu/eid_qa.htm

Questions and Answers: EID article "Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April-July 2009"

October 28, 2009, 4:30 PM ET

Summary

Through July 2009, a total of 43,677 laboratory-confirmed cases of 2009 H1N1 were reported in the United States, which is likely a substantial underestimate of the true number. Correcting for under-ascertainment using a multiplier model, researchers in this study estimate there may have been between 1.8 million and 5.7 million cases during this time period, including 9,000-21,000 hospitalizations. This article is available online at Emerging Infectious Diseases online:

http://www.cdc.gov/eid/content/15/12/pdfs/09-1413.pdf

(hat tip Avian Flu Diary)

[indigo girl]

Swine flu: How Bad Was the First Wave?

The Reveres commentary on Estimates of the Prevalence of Pandemic (H1N1) 2009, United States,

April-July 2009

One frequently hears claims that the current swine flu pandemic has been exaggerated because there are "only" 1000 or so deaths, while seasonal flu is estimated to contribute to tens of thousands of deaths a year. There are two reasons why this is not an apt comparison. We've discussed both here fairly often. The first is that the epidemiology of a pandemic and seasonal flu are very different. Epidemiology studies the patterns of disease in the population and swine flu is hitting -- and killing -- a very different demographic from seasonal flu. Its victims are young and many are vigorous and healthy. The second is that it compares apples to oranges. The 1000 deaths figure is for laboratory confirmed swine flu deaths (as are the various case counts), while the seasonal flu figure is an estimate, not a count of laboratory confirmed influenza deaths (see our post here if you want to know more about how the sausage is made). CDC and the states stopped counting cases early in the pandemic (here for some commentary from us), so we don't know how many cases there have really been. CDC keeps track of the general trends and patterns through a multi-part surveillance system. But for planning and resource allocation it would still be nice to know how much flu there is. Now a paper has appeared in Emerging Infectious Diseases that provides us with some rough and ready estimates. It also explains why this number is so hard to get.

Full commentary here: http://scienceblogs.com/effectmeasure/2009/10/how_bad_was_the_first_wave.php