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. 2017 Jun;17(6):645-653.
doi: 10.1016/S1473-3099(17)30111-1. Epub 2017 Feb 28.

Asymptomatic infection and unrecognised Ebola virus disease in Ebola-affected households in Sierra Leone: a cross-sectional study using a new non-invasive assay for antibodies to Ebola virus

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Asymptomatic infection and unrecognised Ebola virus disease in Ebola-affected households in Sierra Leone: a cross-sectional study using a new non-invasive assay for antibodies to Ebola virus

Judith R Glynn et al. Lancet Infect Dis. 2017 Jun.

Abstract

Background: The frequency of asymptomatic infection with Ebola virus is unclear: previous estimates vary and there is no standard test. Asymptomatic infection with Ebola virus could contribute to population immunity, reducing spread. If people with asymptomatic infection are infectious it could explain re-emergences of Ebola virus disease (EVD) without known contact.

Methods: We validated a new oral fluid anti-glycoprotein IgG capture assay among survivors from Kerry Town Ebola Treatment Centre and controls from communities unaffected by EVD in Sierra Leone. We then assessed the seroprevalence of antibodies to Ebola virus in a cross-sectional study of household contacts of the survivors. All household members were interviewed. Two reactive tests were required for a positive result, with a third test to resolve any discrepancies.

Findings: The assay had a specificity of 100% (95% CI 98·9-100; 339 of 339 controls tested negative) and sensitivity of 95·9% (89·8-98·9; 93 of 97 PCR-confirmed survivors tested positive). Of household contacts not diagnosed with EVD, 47·6% (229 of 481) had high level exposure (direct contact with a corpse, body fluids, or a case with diarrhoea, vomiting, or bleeding). Among the contacts, 12·0% (95% CI 6·1-20·4; 11 of 92) with symptoms at the time other household members had EVD, and 2·6% (1·2-4·7; 10 of 388) with no symptoms tested positive. Among asymptomatic contacts, seropositivity was weakly correlated with exposure level.

Interpretation: This new highly specific and sensitive assay showed asymptomatic infection with Ebola virus was uncommon despite high exposure. The low prevalence suggests asymptomatic infection contributes little to herd immunity in Ebola, and even if infectious, would account for few transmissions.

Funding: Wellcome Trust ERAES Programme, Save the Children.

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Conflict of interest statement

Declaration of interests

Save the Children International operated the Kerry Town Ebola Treatment Centre during the period under study, and employed SJ, MSB, AJK, OK, SHM, DS, and CT. FC was employed by Save the Children UK and was involved in commissioning the study and interpreting the findings. The authors declare no other competing interests.

Figures

Figure 1
Figure 1. Flow chart of study participants
Households were defined as those who ate from the same pot. They included everyone who stayed there at the time Ebola was in the household, including those who were not normally resident. Of those not swabbed, most were absent; eight refused (all had been asymptomatic) and four had died since Ebola. Of the six excluded swabs, three were miscoded and three were not found. EVD=Ebola virus disease.
Figure 2
Figure 2. Normalised optical densities of the first test in samples from 116 Kerry Town survivors and 339 Sierra Leone controls
Figure 3
Figure 3. Ebola manifestation and risk in households of survivors of Ebola virus disease (A) by age group in all members and (B) by exposure level (excluding the primary cases in each household)
The primary cases were excluded for (B) so that the outcomes for each type of contact in Ebola-affected households could be seen. Information on deceased household members was provided at interview by the surviving household members. Exposure levels were determined from the interviews with all household members. Exposure levels are defined as follows: corpse, touched body of someone who died of EVD; fluids, direct contact with body fluids of a wet case (ie, an EVD case with diarrhoea, vomiting, or bleeding); direct wet, direct contact with a wet case (including nursing and personal care, sharing a bed, breastfeeding an EVD-positive child); direct dry, direct contact with a dry case (ie, an EVD case without wet symptoms); indirect wet, indirect contact with a wet case (eg, washing clothes or bed linen); indirect dry, indirect contact with a dry case; minimal or none, minimal contact (eg, shared meals) or no known contact. See Bower and colleagues for details. EVD=Ebola virus disease.

Comment in

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