Tracing the potential extra-household contacts of TB patients: findings from a personal social network survey in a high TB burden setting in India
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Abstract:
Evidence on the extra-household contacts of TB patients who drive disease transmission is scarce.We conducted a cross-sectional personal social network survey among 300 newly diagnosed index pulmonary TB patients to identify their first-degree extra-household contacts.A significantly higher proportion of neighbourhood (3.5; 95% CI 1.3 to 7.5), occupational (3.2; 95% CI 1.3 to 9.2) and friendship contacts (2.2; 95% CI 0.8 to 4.5) developed TB within 1 y of the index patient's diagnosis than their household contacts (0.7; 95% CI 0.3 to 1.3). Similarly, a higher proportion of extra-household contacts had TB at different time points before the index patient was diagnosed.Extra-household contacts of TB patients could be a potential source of TB or could be at increased risk of TB.Keywords:
Contact tracing
Neighbourhood (mathematics)
Cross-sectional study
Index case
Contact tracing and investigation can prevent infectious disease transmission when carried out promptly. Data obtained from contact tracing also provides unique information on transmission dynamics that can be useful for policy making. The proportion of secondary infections arising from index cases that were previously identified as a close contact, hence “known index cases”, has been indicated by ECDC as a key performance indicator for contact tracing. Few empirical studies have been published on the effectiveness of contact tracing and investigation during the COVID-19 pandemic.
Using data from the Belgian COVID-19 contact tracing (Sept 2020 – Dec 2021), we investigated the impact of contact tracing on onward transmission. Known index cases were compared to those not previously identified as close contact. More specifically, we compared their traced contacts and secondary cases as well as the serial interval, representing the time from illness onset in the primary case to illness onset in the secondary case. In addition, we investigated contact and transmission characteristics by age.
Known index cases, previously informed about their exposure, were linked to fewer high-risk contacts and secondary infections. In addition, they presented a lower secondary attack rate compared to those not previously identified by contact investigation, suggesting behavioural changes. Also, a shorter serial interval was observed for known index cases. In addition to breaking transmission chains, contact tracing provides important information on transmission characteristics. When exploring age-specific data, we observed increased transmission rates from children to adults during the circulation of Delta and Omicron variants without an increase in contact rates between these age groups.
The lower secondary attack rates for known index cases suggest that contact tracing in Belgium has been effective in reducing onward transmission. Their lower number of traced contacts and shorter serial interval are indicative that individuals aware of their exposure to SARS-CoV-2 were more reserved in their social contact behaviour. This reflects the mandatory quarantine for high-risk contacts. Our study also shows that in times of high burden on the healthcare system, key performance indicators of contact tracing should be interpreted with caution in light of changing intervention strategies. Many external factors influence the effectiveness of contact tracing like testing policy and compliance, the cooperation of index cases, and the imposed and adopted quarantine and isolation policies.
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In an urban city, a decision was taken in 2004 to modify tuberculosis (TB) contacts screening strategy from targeting only close contacts identified by interview with the index case (the national policy) to a screening strategy involving home and work visit. The present study compares TB contact-tracing during the periods 2001 to 2003 and 2004 to 2006 trying to capture how the new contact tracing strategy addressed the three main questions: (i) Does this strategy increase TB screening compliance? (ii) Does it identify more at-risk contacts? (iii) Does it result in more TB cases being prevented? Home and work visit allowed to identify more at risk contacts (8.4/index case) than the interview (2.5/index case); to improve compliance (87.3% of the identified contacts were screened, compared to 67.6%). More cases of active TB and LTBI cases were detected (1.4/index case compared with 0.75/index case) and prevented more TB cases in the future. The newly implemented contact-screening program with home and work place evaluation of active TB patients, improved compliance to screening procedures, identified more at-risk contacts and should allow us to prevent more TB cases in the future.
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A 25-year-old woman, later identified as index case of Crimean-Congo hemorrhagic fever (CCHF), presented to Holy Family Hospital in Rawalpindi, Pakistan with fever and generalized coagulopathy. A retrospective contact tracing was conducted to explore the modes of exposure possibly associated with transmission of CCHF infection among contacts. We traced 32 contacts of the index case and 158 contacts of secondary cases and tested them for IgG and IgM antibodies against CCHF virus by an enzyme-linked immunosorbent assay technique. According to the type of exposure, contacts were divided into five subsets: percutaneous contact with blood, blood contact to unbroken skin, cutaneous contact to non-sanguineous body fluids, physical contact with patients without body fluids contact, and close proximity without touching. Two out of four contacts who reported percutaneous exposure tested positive for antibodies to CCHF virus. We conclude that simple barrier methods and care in provision of CCHF cases may prevent transmission of this infection.
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Crimean–Congo hemorrhagic fever
Body contact
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An outbreak of the SARS-CoV-2 Delta variant occurred in Guangzhou in 2021. This study aimed to identify the transmission dynamics and epidemiological characteristics of the Delta variant outbreak to formulate an effective prevention strategy.A total of 13102 close contacts and 69 index cases were collected. The incubation period, serial interval, and time interval from the exposure of close contacts to the symptom onset of cases were estimated. Transmission risks based on the exposure time and various characteristics were also assessed.The mean time from exposure to symptom onset among non-household presymptomatic transmission was 3.83 ± 2.29 days, the incubation period was 5 days, and the serial interval was 3 days. The secondary attack rate was high within 4 days before onset and 4-10 days after symptom onset. Compared with other contact types, household contact had a higher transmission risk. The transmission risk increased with the number and frequency of contact with index cases. Cycle threshold (Ct) values were associated with lower transmission risk (adjusted odds ratio [OR] 0.93 [95% CI 0.88-0.99] for ORF 1ab gene; adjusted OR 0.91 [95% CI 0.86-0.97] for N gene).The contact tracing period may need to be extended to 4 days before symptom onset. The low Ct value of index cases, the high number and frequency of contact with index cases, and household contacts were associated with a higher transmission risk of SARS-CoV-2 Delta.
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Abstract Background We aimed to quantify the risk of transmission of SARS-CoV-2 in the school setting by type of school, characteristics of the index case and calendar period in the Reggio Emilia province (RE), Italy. The secondary aim was to estimate the promptness of contact tracing. Methods A population-based analysis of surveillance data of all COVID-19 cases occurring in RE, Italy, from September 1, 2020, to April 4th, 2021, for which a school contact and/or exposure was suspected. Indicator of the delay in contact tracing was computed as the time elapsed since positivity of the index case and the date on which the swab for classmates was scheduled (or most were scheduled). Results Overall, 30,184 and 13,608 contacts among classmates and teachers/staff, respectively, were identified and received recommendation for testing; 43,214 (98.7%) performed the test. Secondary transmission occurred in about 40% of the investigated classes, and the overall secondary case attack rate was 4%, slightly higher when the index case was a teacher, but with almost no differences by type of school and stable during the study period. Promptness of contact tracing increased during the study period, reducing the time from index case identification and testing of contacts from 7 to 3 days, as well the ability to identify possible source of infection in the index case. Conclusions Despite the spread of the Alpha variant during the study period in RE, the secondary case attack rate remained stable from school reopening in September 2020 until the beginning of April 2021.
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The extent to which vaccinated persons diagnosed with coronavirus disease 2019 (COVID-19) can transmit to other vaccinated and unvaccinated persons is unclear.
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Contact tracing
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Infectivity
Asymptomatic carrier
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Background The role of school contacts in the spread of the virus and the effectiveness of school closures in controlling the epidemic is still debated. We aimed to quantify the risk of transmission of SARS-CoV-2 in the school setting by type of school, characteristics of the index case and calendar period in the Province of Reggio Emilia (RE), Italy. The secondary aim was to estimate the speed of implementation of contact tracing. Methods A population-based analysis of surveillance data on all COVID-19 cases occurring in RE, Italy, from 1 September 2020, to 4 April 2021, for which a school contact and/or exposure was suspected. An indicator of the delay in contact tracing was calculated as the time elapsed since the index case was determined to be positive and the date on which the swab test for classmates was scheduled (or most were scheduled). Results Overall, 30,184 and 13,608 contacts among classmates and teachers/staff, respectively, were identified and were recommended for testing, and 43,214 (98.7%) underwent the test. Secondary transmission occurred in about 40% of the investigated classes, and the overall secondary case attack rate was 4%. This rate was slightly higher when the index case was a teacher but with almost no differences by type of school, and was stable during the study period. Speed of implementation of contact tracing increased during the study period, with the time from index case identification to testing of contacts being reduced from seven to three days. The ability to identify the possible source of infection in the index case also increased. Conclusions Despite the spread of the Alpha variant during the study period in RE, the secondary case attack rate remained stable from school reopening in September 2020 until the beginning of April 2021.
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Transmission rate
Attack rate
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