National Influenza Center – Zambia
Pathology and Microbiology Department, University Teaching Hospital, Virology Laboratory
Methodology for Establishment of Epidemic Thresholds
Thresholds are calculated using Moving Epidemic Methods (MEM), a sequential analysis using R language available from: http//
CRAN.R-project.org/web/package =mem) designed to calculate the duration, start and end of the annual influenza epidemic. MEM
uses the 40th, 90th and 97.5th percentile established from available years of historical data to calculate threshold activities. Threshold
activity for influenza is categorized as: below epidemic threshold, low, moderate, high or very high. Transmissibility of influenza
can be inferred from ILI data while SARI data gives an indication of severity.
There was increased influenza activity at the beginning of the 4th quarter of 2019 between epi-weeks 40 and 48. Rates of Influenza-
Like Illness (ILI) and Severe Acute Respiratory Infection (SARI) attributable to influenza virus infection were in the high to moderate
threshold and remained within the moderate seasonal threshold in week 48. This second cycle of activity was of a moderate
transmissibility and low severity.
Specimens from 1070 outpatients were received from two ILI surveillance sites. 1035 (97%) were adequately sampled and tested.
Influenza virus was detected in 232 (22%) of these samples. 86 (37%) were identified as Influenza B, 52 (22%) Influenza A H3N2,
46 (20%) Influenza A H1N1 (pandemic), 38 (16%) influenza A Untyped and 10(4%) as Influenza A unsubtypeable.
During this same period, specimens were received from 1681 patients admitted to four SARI surveillance sites. 1251 (74%) were
adequately sampled and tested. Influenza was detected in 206 (16%) specimens; 119 (58%) of which were identified as Influenza
B, 23(11%) as Influenza A H3N2, 20 (10%) as Influenza A H1N1 (pandemic), 33(16%) influenza A Untyped and 11(5%) as Influenza
Fig 1: Percentage of Influenza Positive ILI Cases1 (Out-Patient Visit Surveillance) per Epi-Week against Epidemic Thresholds Set
Using 2013 – 2018 Data
The total number of samples collected as at 30th November 2019, is 2751. 95% (2702/2751) of the received samples were tested
and 16.2% (438/2702) were positive for influenza virus while 83% (2268/2702) were negative. Half of the total patients investigated
were aged 10 years.