New study reveals casual contact drives nearly half of TB transmission in social settings

7 Apr 2025

For centuries, it was believed that tuberculosis spread primarily when a vulnerable person spent hours in a poorly ventilated space with someone who was infectious. But new findings suggest that much TB transmission also occurs through casual contact.

Conventional thinking held that enclosed spaces such as households, prisons, and shelters, where people spent long periods of time together, were where most TB transmission took place. But new data suggest that casual contact at social settings like shopping malls, restaurants, bars, and places of worship also account for much TB transmission.

A recent study found that close contact explained only 9% of TB transmission links, while casual contact accounted for 49%. The study, called Context (Casual Contact and Migration in XDR TB), was conducted in KwaZulu-Natal.

The study’s lead author, Professor Neel Gandhi of Emory University in Atlanta, recently presented the findings at the Conference on Retroviruses and Opportunistic Infections in San Francisco. The work has not yet been published in a peer-reviewed medical journal.

The new findings come in the context of other research (much of which was conducted in Cape Town) that suggest TB could be transmitted through breathing, and growing evidence that people with asymptomatic TB can transmit the infection.

Where transmission occurs

Gandhi tells Spotlight that TB transmission has traditionally been linked to prolonged, close contact, with previous studies showing that 9 to 30% of cases could be attributed to this type of contact. A compelling alternative argument, he said, was that the remaining 70% of transmission occurs due to casual contact in community settings — which was what their research sought to explore.

He elaborated: “For much of history, we have thought that most TB transmission occurs through close and prolonged contact, meaning that a susceptible person is spending a lot of time in a poorly ventilated area with somebody who is infectious. And so most often we think of households as places where transmission occurs; or congregate settings, places like prisons or homeless shelters.”

On defining casual contact, he said: “In our research, we wanted to understand less intense forms of contact where transmission can occur. So, we understood where people lived, but we also asked them where they spent time in a typical week. The phrase we used was: ‘Where do you spend two hours or more, most weeks?’ To try to identify the places people spend substantial amounts of time; and seeing whether they crossed paths with somebody else to whom their molecular fingerprints (of their TB bacteria infection) matched.”

Genotyping, and geomapping

In their study, Gandhi and his colleagues made use of both genotyping and geospatial mapping to figure out where TB transmission probably occurred.

Genotyping, explained Gandhi, was a technology developed about 30 years ago that allowed us to examine the genetic code of TB bacteria, and to compare similarity between patients’ bacteria.

“TB is a bacteria that keeps its genetic code similar across many generations of replication. In layman’s terms, we call this molecular fingerprinting. If I were to transmit TB to somebody else; my TB bacteria and that person’s TB bacteria’s genetic codes would look very similar — almost identical — so we could use this fingerprinting technique by sequencing the genomes of the two TB bacteria to try to fully get a sense of what the likelihood of transmission was.”

Commenting on their geospatial methodology, he said: “When our participants told us where they live or where they spend time in the community, or where they get outpatient healthcare; our team went to those sites and captured GPS coordinates.

“Just like we use GPS for mapping when we’re trying to get around town, we would get specific coordinates… If two people went to the same shop, they might have used different names for that shop, or let’s say they went to a shopping mall, they may have used different names for those places; but we used GPS coordinates allowing us to determine whether they were at the same place or close to one another. And we used the concept of proximity to try to understand the likelihood that they may have crossed paths.”

In the study they used the metric of “community proximity”, defined as a radius of 500 metres, or less.

Gandhi illustrated the nuance of geomapping, using his university campus: “So the example I like to give is; I work in a building called the School of Public Health. Across the courtyard is the School of Nursing. If you just asked me, where do you work? I would tell you, I work in this building. If you ask the next person where they work, they may say, I work in the School of Nursing. That wouldn't match up in terms of place name. But if we used a radius of 100m or 500m, we can determine that we work very close to one another. And there’s a café in yet another building that we may have eaten lunch in at the same time. TB being an airborne disease, I don’t have to sit next to that person or even to know that person; if I’m infectious, I could have transmitted it to them if they were sitting and eating in the same room.”

Essentially, the researchers used genotyping, particularly molecular fingerprinting, to help understand the likelihood of transmission between people who have drug-resistant TB. And once individuals with similar molecular fingerprints were found, they used geomapping to see whether these patients could be connected through close contact — and if not close contact, then through casual contact.

He added: “The most common place people told us were friends and family members’ homes. Then the next most common was places of shopping, so shopping malls.”

At Conference on Retroviruses and Opportunistic Infections, Gandhi responded to a question from a conference delegate about risk, saying that there appeared to be a greater risk of TB transmission in social settings than previously understood.

Symptoms and disease

To Spotlight, he said more work was needed to understand why casual contact transmission was taking place.

“And it connects to another topic in the TB community that is gaining a lot of attention currently, which is trying to understand what the association is between symptoms and having TB disease,” said Gandhi.

He noted that the challenge for researchers moving forward was understanding the link between infectiousness and symptoms – specifically, understanding when a person became infectious, even if they showed no symptoms.

Most TB public health interventions were still based on the assumption that people with TB would present at health facilities with classic TB symptoms such as persistent cough, night sweats, fever, weight loss, and chest pain. South Africa had, however, in recent years been offering TB tests to asymptomatic people thought to be at high risk of TB, as part of its targeted universal testing strategy.

“So you may have heard of this concept of what some people have called subclinical TB or asymptomatic TB. And that is to say, if you were to test a group of people who didn’t come to a health clinic, but let’s say you were on a street corner and you tested everybody who went by for TB, we’re coming to appreciate that as many as 50% of people may not either have any symptoms, or may not have symptoms that are worrisome enough for them to seek healthcare, but are actually testing positive for TB disease,” Gandhi added.

Gandhi said this reminded him of the early days of Covid-19, when scientists weren’t sure if people only became infectious after showing symptoms.

“Eventually we learned that people were infectious probably for a few days before they developed symptoms. And in the TB world, this may be an area we need to investigate. If there’s the possibility that somebody is infectious when they have absolutely no symptoms, they would go about their regular activities; going to work, going to school, going shopping, going to religious ceremonies, going to restaurants, and they may unknowingly be infectious with TB. So this is the challenge.”

The bigger picture

Commenting on the findings, Robert Wilkinson, Honorary Professor in the Department of Medicine at the University of Cape Town and director of the Centre for Infectious Diseases Research in Africa, said: “It is interesting, and the proportion of transmission estimated to occur outside the household is a low estimate, but not incompatible with other estimates.”

He noted that the phenomenon of transmission occurring after brief casual contact was not novel though, and has been investigated in previous studies.

Asked how the findings presented by Gandhi might affect the outlook on TB interventions, Wilkinson said: “While close household exposure to infectious tuberculosis should prompt clinical evaluation especially if there are symptoms, finding a close contact by conventional contact tracing approaches is far from invariable. Therefore, in high incidence environments like South Africa more attention needs to be placed on mass radiographic (X-ray) and, or microbiological screening of asymptomatic persons.”

In a recent public lecture called “Hunting Bosons, Finding the Bummock”, Emeritus Professor in Medicine at the University of Cape Town, Robin Wood, former CEO of the Desmond Tutu Health Foundation, stated: “I think we are changing the paradigm of tuberculosis.” He noted that research now targeted “hidden reservoirs of TB transmission beyond visible, symptomatic cases… (as) TB silently spreads within communities through carriers who exhibit no symptoms yet contribute to transmission.”

Asked about Gandhi’s findings, Wood told Spotlight he would reserve comment until the data was submitted for further peer review and publication.

Study details

The 305 respondents in the Context study were patients with extensively drug-resistant TB or pre-extensively drug-resistant TB. They were diagnosed between 2019 and 2022 in the eThekwini, Ilembe, Umgungundlovu, and Ugu regions. The average age was 36 years, with 137 (45%) women and 216 (73%) people living with HIV.

The study was conducted in collaboration with the Durban-based Centre for the Aids Programme of Research in South Africa (Caprisa).

“Caprisa played a leadership role in conceptualising the science, development of the protocol and data collection instruments, oversight of all aspects of field work, including screening and enrolling patients, obtaining informed consent from patients or their proxies, field and laboratory data collection, data verification and data clean-up activities for all data used in this study,” said Caprisa’s deputy director, Professor Kogieleum Naidoo.

Context was funded through the United States National Institutes of Health (NIH), the world’s largest health research funder, which has in recent weeks terminated several grants in South Africa and elsewhere.

“The funding period has ended,” said Gandhi. “Now we’re analysing all of the data, so it won’t be affected by any changes happening at NIH.” DM

This article was first published by Spotlight – health journalism in the public interest. Sign up to the Spotlight newsletter.

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South Africa

Where transmission occurs

Genotyping, and geomapping

Symptoms and disease

The bigger picture

Study details