Table 1.
Key features of case–control studies conducted in response to gastrointestinal outbreaks using market research panel controls, UK, 2013–2016
| Study Identifier (ref) | A [1, 2] | B [3] | C [5] | D [6] | E [6] | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Year | 2013 | 2014 | 2015 | 2015 | 2016 | ||||||||||
| Pathogen | Salmonella Mikawasima | STEC O157 PT8 | Salmonella Enteritidis (5 SNP cluster) | Cryptosporidium parvum IIdA24G1 | STEC O157 PT34 | ||||||||||
| Hypothesised exposure from trawl (noted excess of demographic groups) | A large proportion of cases reported taking PPIs prior to illness but no clear exposure associated with illness was identified. No excess of cases noted in defined age or gender groups | Handling or consumption of potatoes, root vegetables, Tomatoes, apples or bananas. No excess of cases noted in defined age or gender groups | Exposure to reptiles, particularly snakes. Excess of cases in children | Consumption of pre-prepared sandwiches with specific fillings, food bought from branches of two supermarkets and one coffee shop chain A, specific dairy products or consumption and/or handling of specified salad vegetables. Excess of cases in adult females | Consumption of salad vegetables, bagged salad, food purchased from a specific supermarket chain and salad items from catering premises. Excess of cases in adult females | ||||||||||
| Method of collecting case data | Telephone interview | Web-survey | Telephone interview | Telephone interview | Telephone interview | ||||||||||
| Target control criteriaa | Age restricted to 18 years and over. Frequency-matched on sex and investigating region. Intended 2 : 1 control-to-case ratio | Age restricted to 18 years and over. Intended 2 : 1 control-to-case ratio | Frequency matched on age (including households with children) and sex. Intended 3 : 1 control-to-case ratio | Age restricted to 20 years and over. Frequency matched on country. Intended 2 : 1 control-to-case ratio | Age restricted to 18 years and over. Frequency matched on sex and geographical area. Intended 4 : 1 control-to-case ratio | ||||||||||
| Panel (X or Y) | X | X | X | X | Y | ||||||||||
| Number of controls (and cases) included in study | 123b (39c) | 96 (36c) | 180b (28c) | 133b (52c) | 91b (21) | ||||||||||
| Number of survey invitations distributed (% included in study) | 1329 (9%) | Data not available | Data not available | Data not available | 4772 (2%) | ||||||||||
| Price per control (£) | £3.60 | £3.20 | £3.00 | £3.30 | £2.00 | ||||||||||
| Time to organise with company (to point of survey distribution) | 18 days | 5 days | 7 days | 2 days | 2 days | ||||||||||
| Time to recruit and collect data (from survey distribution) | <1 day (9 h) | 7 days | 14 days | 2 days | <1 day (10 h) | ||||||||||
| Means of distributing survey | By email | By email | By email | By email | Survey invitations delivered directly to targeted individuals’ home page for the panel and notified of invite by email | ||||||||||
| Distributions | Group | Cases (%) | Controls (%) | Group | Cases (%) | Controls (%) | Group | Cases (%) | Controls (%) | Group | Cases (%) | Controls (%) | Group | Cases (%) | Controls (%) |
| 1. Age | 18–25 years | 6 (15) | 5 (4) | 18–30 years | 5 (14) | 11 (11) | <10 years | 9 (32) | 0 (−) | 20–29 years | 23 (44) | 7 (5) | 10–19 years | 1 (5) | 0 (0) |
| 26–35 years | 8 (21) | 19 (16) | 31–64 years | 16 (44) | 63 (66) | 10–19 years | 6 (21) | 0 (−) | 30–39 years | 15 (29) | 23 (17) | 20–29 years | 5 (24) | 2 (2) | |
| 36–45 years | 6 (15) | 23 (19) | 65 + years | 15 (42) | 22 (23) | 20–29 earsy | 7 (35) | 14 (8) | 40–49 years | 13 (25) | 29 (22) | 30–39 years | 1 (5) | 7 (8) | |
| 46–55 years | 9 (23) | 40 (33) | – | – | – | 30–39 years | 2 (7) | 53 (29) | 50–59 years | 0 (−) | 45 (34) | 40–49 years | 4 (19) | 21 (23) | |
| 56–65 years | 10 (26) | 36 (29) | – | – | – | 40–49 years | 3 (11) | 66 (37) | 60–69 years | 1 (2) | 29 (22) | 50–59 years | 4 (19) | 22 (24) | |
| – | – | – | – | – | 50–59 years | 1 (4) | 38 (21) | – | – | – | 60–69 years | 1 (5) | 27 (30) | ||
| – | – | – | – | – | 60–69 years | 0 (−) | 7 (4) | – | – | – | 70–79 years | 3 (14) | 12 (13) | ||
| – | – | – | – | – | 70–79 years | 0 (−) | 2 (1) | – | – | – | >80 years | 2 (10) | 0 (0) | ||
| P-valued | 0.124 | 0.068 | <0.001 | <0.001 | <0.001 | ||||||||||
| 2. Sex | F | 22 (56) | 63 (51) | F | 29 (81) | 43 (45) | F | 11 (39) | 90 (50) | F | 31 (60) | 54 (41) | F | 19 (90) | 77 (85) |
| M | 17 (44) | 60 (49) | M | 7 (19) | 53 (55) | M | 17 (61) | 90 (50) | M | 21 (40) | 79 (59) | M | 2 (10) | 14 (15) | |
| P- valued | 0.572 | <0.001 | 0.293 | 0.020 | 0.768e | ||||||||||
| 3. Geographical | Scotland | 9 (23) | 48 (39) | – | – | – | – | – | – | England | 44 (84.6) | 109 (82) | South East England | 5 (24) | 21 (23) |
| North East England | 13 (33) | 32 (26) | – | – | – | – | – | – | Scotland | 4 (7.7) | 12 (9) | South West England | 16 (76) | 70 (77) | |
| West Midlands England | 9 (23) | 22 (18) | – | – | – | – | – | – | Wales | 4 (7.7) | 12 (9) | – | – | – | |
| London | 8 (21) | 21 (17) | – | – | – | – | – | – | – | – | – | – | – | – | |
| – | – | – | – | – | – | – | – | – | – | – | – | ||||
| P-valued | 0.344 | – | – | 0.185 | >0.999e | ||||||||||
| Study finding | Consumptions of a variety of chicken exposures, coriander, salad garnishes and eating out were significantly associated with illness | Consumption of pre-packed salad, and purchase of salad or potatoes from a specific retailer were independently associated with illness | Snake ownership was independently associated with illness | Coffee shop chain A was independently associated with illness | Mixed leaves and ‘other’ salad items associated independently with illness | ||||||||||
a
Intention to recruit controls based on number of cases expected to be included in the study.
b
The target quota of recruited controls was exceeded before the web-survey was closed.
c
Some cases could not be contacted, were not eligible or did not agree to be interviewed.
d
Evidence of differences in distribution (at 5% level) between cases and controls.
e
Fishers exact test (all others Chi-squared test).