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Incidence details
MonthIllness episodesPerson-days at riskIncidenceMembers participating
May 201154091.2263
June 20113425.60.7043
July 20112428.50.4735
August 20114495.70.8139
September 20113583.00.5147
October 20116535.71.1236
November 20114519.00.7734
December 20116863.40.6987
January 20124937.30.4366
February 20122913.60.2258
March 20127935.60.7559
April 20124890.30.4557
May 20124786.00.5151
June 20121740.60.1446
July 20123716.10.4246
August 20122661.30.3045
September 20125666.60.7550
October 20126646.00.9352
November 20123557.90.5441
December 20123541.30.5537
January 20134557.30.7240
February 20133557.50.5439
March 20133608.30.4949
April 20133546.00.5542
May 20134613.10.4941
Influenza-like illness
MonthIllness episodesPerson-days at riskIncidenceMembers participating
May 20112186.31.0735
June 20111266.30.3829
July 20110247.30.0021
August 20111254.70.3920
September 20110323.70.0026
October 20110290.30.0017
November 20111444.10.2331
December 20113876.40.3486
January 20122946.20.2166
February 20121902.50.1157
March 20126924.80.6559
April 20121892.60.1156
May 20123756.00.4051
June 20121703.70.1445
July 20121731.10.1446
August 20121662.80.1545
September 20123672.40.4550
October 20125643.80.7851
November 20121640.80.1840
December 20122537.60.3736
January 20132519.50.3938
February 20131569.10.1838
March 20133588.10.5148
April 20132526.00.3842
May 20132610.10.3341
Common cold
MonthIllness episodesPerson-days at riskIncidenceMembers participating
May 201111112.19.8132
June 20119165.65.4425
July 20115151.83.2918
August 20112167.21.2016
September 20114190.02.1119
October 20113193.01.5515
November 20113187.61.6016
December 201111340.73.2340
January 20128414.91.9334
February 20127362.31.9327
March 20127393.81.7828
April 20127473.91.4831
May 20122367.50.5426
June 20125321.01.5623
July 20121307.30.3321
August 20121312.50.3220
September 20126317.61.8925
October 20123231.51.3021
November 20124204.51.9621
December 20124154.32.5913
January 20136175.43.4218
February 20136236.82.5320
March 20134219.31.8221
April 20134202.71.9719
May 20134250.01.6019
MonthIllness episodesPerson-days at riskIncidenceMembers participating
May 20117237.82.9440
June 20113158.51.8919
July 20114215.71.8517
August 20114286.31.4023
September 20118407.11.9629
October 20118383.72.0925
November 20117450.01.5636
December 201112785.71.5388
January 201212861.61.3966
February 201214845.91.6659
March 201213866.51.5059
April 201216812.51.9757
May 201217681.62.4951
June 201211665.51.6546
July 20128669.11.2046
August 20127606.51.1545
September 20128634.01.2651
October 20128632.91.2652
November 20129501.51.7940
December 201212485.02.4737
January 20138523.41.5340
February 20134561.50.7138
March 201312558.72.1549
April 20135524.80.9542
May 20139571.31.5841

summary updated on June 5, 2013



All members were asked about fever. Until December, 2011, most members were asked either about respiratory or gastrointestinal symptoms, but not both types of symptoms. Beginning in December, 2011, nearly all members were asked about fever, cough, vomiting, and diarrhea, as well as other respiratory or gastrointestinal symptoms.

Illness episode
An illness episode is a period during which a member reported illness symptoms (for self or child) in at least one symptom update (with a recall period ranging between 20 and 72 hours). Members could have more than one illness episode during a month.

Reports of illness symptoms less than 7 days apart were considered to be part of the same illness episode, even if a member reported no corresponding symptoms in the intervening period. Reports of illness symptoms less than 14 days apart, if a member had not completed any symptom updates in the intervening period, were considered to be part of the same illness episode. If a member reported only one symptom of influenza-like illness or a common cold illness subsequent to an earlier report that fully satisfied the illness definition, the subsequent report(s) was considered to be part of the same influenza-like illness episode if it met these criteria.

Person-days at risk
Person-days at risk refer to the days a person is at risk for being ill (that is, days on which a person is not already ill, or had not been recently ill, according to the definition of illness episode). One day for 10 persons and 10 days for one person both equal 10 person-days. The vertical axis in the graph on the Results page shows the number of illnesses per 100 person-days at risk. This can be interpreted as a percentage of members who became ill on the typical day that month.

Members participating
The number of members participating is the number of members who completed one or more symptom updates that month.

These indicidence rates are based on a small number of members for a large and diverse geographic area, and should not be treated as generalizable estimates. These rates are presented here only to illustrate the incidence measure and trends among members. As with nearly all measures of crude disease incidence in open cohorts, the measure here is based on the assumption that incidence is not associated with the frequency of members' participation (length of observation) or with the changing composition of participating members over time.

As more members join and participate regularly, we will calculate incidence rates for shorter and more recent time periods, smaller geographic areas, and additional illnesses and symptoms.

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