# 3902
For several months the CDC has been promising to come out with better estimates on the early impact of the H1N1 virus. Given a society of more than 300 million people, and limited surveillance and reporting capabilities, that is no easy task.
Today, however, we get a study in the EID Journal that attempts to do just that.
Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009
C. Reed et al. (364 KB, 7 pages)
All along, it has been assumed that the number of infections, hospitalizations, and even deaths from the pandemic virus were under counted. That’s a subject we’ve discussed many times in the past, including here, here, here, and here.
The numbers reported by the CDC over the summer were of laboratory confirmed cases, only.
While an accurate `count’ is impossible, we now have some estimated ranges. The entire study is well worth reading, but we’ll jump to the conclusions, to see what these researchers have come up with.
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).
To recap: Over a time period (April-July 23rd) when the official numbers were:
43,677 confirmed cases
5009 hospitalizations
302 deaths
The newly estimated numbers are:
1.8 – 5.7 Million cases
9,000-21,000 Hospitalizations
800 (range 550-1300) deaths
Obviously, since this study only goes to July 23rd, these numbers are much higher today.
The CDC has posted a Q&A about this study on their website, which should answer some of your questions about this research.
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.
Questions & Answers
What was the main purpose for conducting this study?
It is likely that the 43,677 laboratory-confirmed cases of 2009 H1N1 reported between April and July of 2009 are a substantial underestimation of the true number of cases for this time period. The current study, which used a relatively quick and simple approach, was conducted to help estimate the true number of cases, and the human health impact of 2009 H1N1 during the first four months of the pandemic.
How was this study conducted?
To estimate the total number of cases from April-July 2009, researchers built a probabilistic multiplier model that adjusts the count of laboratory-confirmed cases for each of the following steps: medical care seeking, specimen collection, submission of specimens for confirmation, laboratory detection of 2009 H1N1, and reporting of confirmed cases.
This statistical model was based on a widely accepted technique that has been used previously to estimate the actual number of cases of food-borne illness. 1This model collected information on the number of 2009 H1N1 laboratory-confirmed cases, hospitalizations and deaths in the United States reported to CDC from April – July 2009. These numbers were then adjusted using multipliers to correct for factors that can lead to under-counting. These multipliers and adjustments were made based on analysis of community surveys, outbreak investigations and published data.
What did the study’s findings indicate?
Using the model, researchers estimate that from April-July 2009 the number of people infected with 2009 H1N1 may have been up to 140 times greater than the reported number of laboratory confirmed cases. They estimate that between 1.8 million and 5.7 million cases, including 9,000 – 21,000 hospitalizations, may have occurred during the time period. This indicates that every case of 2009 H1N1 reported from April – July represents an estimated 79 total cases, and every hospitalized case reported may represent a median of 2.7 total hospitalized persons.
