Table 1.

Results From Studies That Estimated Infection Rates for H1N1 Influenza A, 2009

First Author, Year (Reference No.)	Study Location	Study Period	Estimated Infection Rate	Method of Estimation	Details
Lipsitch, 2009 (7)	Mexico	April 2009	0.11%–0.35% during the month of April 2009 (population of 106,682,518)	Surveillance data from travelers	International public health records surveyed to estimate infection rates among travelers to Mexico
					Cases among Mexican residents = cases in travelers × (Mexican population × 30 days)/(traveler population × duration of travel)
D'Ortenzio, 2010 (8)	Réunion Island, France	May 2009–September 2009	12.85% (104,067/810,000)	Sentinel physician network, cross-sectional ARI prevalence survey	Incidence of ARI consultations gathered from social insurance data, adjusted by the proportion of sentinel physician consultations
					Health-care-seeking behavior in persons with ARI from a cross-sectional survey
					Calculated by extrapolating the proportion of randomly selected ARI patients testing H1N1-positive in the total estimated no. of ARI cases
Dawood, 2010 (9)	Hunter New England, Australia	June 1, 2009–August 30, 2009	6.2% (range, 4.4%–8.2%)	Syndromic surveillance and laboratory data	Incidence of ILI from an online self-reporting ILI surveillance system
			53,383 (range, 37,828–70,597) out a population of 866,565		Proportion of ILI samples that tested H1N1-positive from national laboratories
					Using these data, the proportion of ILI cases due to H1N1 was estimated and extrapolated to the general population.
Gordon, 2010 (10)	Nicaragua	June 1, 2009–November 15, 2009	20.1% among children aged 2–14 years	Syndromic surveillance, laboratory testing	Cohort of children selected from an existing dengue study
					Testing criteria were fever with cough, sore throat, or rhinorrhea
					Samples were tested by RT-PCR to determine the H1N1 clinical attack rate.
					No extrapolation to the general population was done.
Flahault, 2009 (5)	France	September 2009–December 2009	10.6% among pregnant women	Cross-sectional seroprevalence	Cross-sectional seroprevalence study from serum obtained from pregnant women in weeks 48–49 of 2009
			1,712,000 cases (95% CI: 1,112,700, 2,311,300) in persons aged 20–39 years		Cumulative seroprevalence was then estimated for the population aged 20–39 years.
Moghadami, 2010 (11)	Iran	December 2009	58.9% (1,504/2,553)	Cross-sectional seroprevalence	Single-sample cross-sectional seroprevalence study
					Serum samples from randomly selected participants in the community
Miller, 2010 (12)	England, United Kingdom	August 2009–September 2009	Age group, years	Cross-sectional seroprevalence	Cross-sectional seroprevalence study involving pre- and postpandemic samples from blood collected for other purposes
			<5: 21.3% (95% CI: 8.8, 40.3)
			5–14: 42.0% (95% CI: 26.3, 58.2)		Infection rates were estimated by subtracting prepandemic seroprevalence from postpandemic seroprevalence.
			15–24: 20.6% (95% CI: 1.6, 42.4)
			25–44: 6.2% (95% CI: −2.8, 18.7)
			45–64: −2.7% (95% CI: −10.3, 7.1)
			≥65: 0.9% (95% CI: −8.8, 13.3)
Chan, 2010 (13)	Taiwan, Republic of China	October 2009–November 2009	30.8% among health-care workers	Cross-sectional seroprevalence	Single-sample cross-sectional seroprevalence study
			12.6% among controls		Serum samples taken from hospital staff and controls
Ross, 2010 (14)	Pittsburg, Pennsylvania, United States	Mid-November–early December 2009	21% (unadjusted)	Cross-sectional seroprevalence	Cross-sectional seroprevalence study with pre- and postpandemic samples
			Range from 5% for persons aged 70–79 years to 45% for persons aged 10–19 years		Prepandemic samples only from young adults aged 18–24 years
			Baseline 6% among young adults aged 18–24 years		Postpandemic samples from laboratory specimens collected for other purposes over a wide age range
Allwinn, 2010 (15)	Germany	November 2009	12% (27/225) with titer of ≥1:40 (unadjusted)	Cross-sectional seroprevalence	First sample from blood donors previously recruited for a serum survey of the spread of enterovirus 71 infection
			Baseline 13.1% (19/145) with titers of 1:>32		Second sample from randomly selected patients at a local university hospital
Grills, 2010 (16)	Australia	August 2009–October 2009	10% in adults aged 18–65 years	Cross-sectional seroprevalence	Participants in a health monitoring program were tested opportunistically.
					Baseline prepandemic seropositive rate from another study was subtracted from the result.
Chen, 2010 (17)	Singapore	June 22, 2009–October 15, 2009	13.5% in community-dwelling adults	Serologic cohort study	Multisample seroepidemiologic cohort study
			6.5% in hospital staff		Serial serum samples from individuals
			29.4% in military personnel		Seroconversion was determined by a 4-fold rise in titers.
			1.2% in long-term-care patients
Crum-Cianflone, 2009 (18)	San Diego, California, United States	April 21, 2009–May 8, 2009	0.53% (101 per 100,000) from April 21, 2009, to May 8, 2009	Complete testing of ILI cases	Complete RT-PCR testing of all ILI cases from a captive population of local US military beneficiaries
Colizza, 2009 (19)	Mexico	April 2009	0.11%–1.31% (121,000–1,394,000 cases as of April 30, 2009)	Mathematical modeling	Model with a geographically structured metapopulation approach
					Use of a population-level census, human mobility flows, and disease dynamics to model disease evolution and infections
Presanis, 2009 (20)	Milwaukee, Wisconsin, and New York, New York, United States	April 2009–July 2009	Not shown; used as a denominator to determine hospitalization and case-fatality rates	Mathematical modeling	Estimation using mathematical model and probabilities of ILI with consultations, consultations that were tested, and proportion positive.
				Data from physician consultations, laboratory, and telephone survey	For New York, a telephone survey was conducted to determine self-reported ILI status.

Abbreviations: ARI, acute respiratory illness; CI, confidence interval; ILI, influenzalike illness; RT-PCR, reverse-transcriptase polymerase chain reaction.