| Peer-reviewed | Observational study | People |
Researchers have provided evidence for asymptomatic transmission of whooping cough, particularly between mothers and their infants, potentially explaining how the illness circulates even in highly vaccinated populations.
The study, published today as a Reviewed Preprint in eLife and described as important work by the editors, investigates transmission of pertussis (commonly known as whooping cough) in a cohort of mothers and infants over the course of a year. The findings challenge traditional surveillance methods that focus only on symptomatic cases, by showing that minimally symptomatic or symptom-free mothers may act as a hidden reservoir of pertussis transmission to their infants. The editors describe the evidence as convincing and say the study is a major contribution to infectious disease epidemiology, particularly in low or middle-resource settings where disease burden is typically higher.
Pertussis is a serious respiratory infection caused by the bacterium Bordetella pertussis (B. pertussis). It is most severe in infants, particularly those under six months old. Early symptoms can resemble the common cold, but after around a week develop into long, intense coughing fits that make breathing difficult. The alternative name ‘whooping cough’ comes from the gasping sound people make as they try to catch their breath in between coughs.
Thanks to widespread vaccination, the rates and severity of pertussis infections fell sharply during the 20th century. However, complete local elimination of the disease has rarely been achieved, unlike for other vaccine-preventable diseases such as measles and rubella. In recent years, cases have started to rise again, even in highly vaccinated populations.
“There’s a lot of debate over the major factors that allow pertussis to persist,” says lead author Christian Gunning, a senior research associate at the Odum School of Ecology, University of Georgia, US. “One key question that we’re beginning to address is how often pertussis infections have mild or no symptoms, and how transmissible these infections are. Historically, pertussis surveillance has focused on symptomatic cases. We tried to develop a more complete picture here using routine surveillance of an at-risk population of newborns and their mothers.”
Gunning and colleagues set out to shed light on this ‘dark matter’ of asymptomatic pertussis transmission by tracking mild and asymptomatic cases during a year-long surveillance study in Lusaka, Zambia. In earlier work, the team had found that many infants showed signs of pertussis infection despite having only mild or no symptoms. In the current study, they dug deeper into the timing and duration of infections, and whether an infection in a mother could predict infection in her baby – or vice versa.
They followed 1,315 mother-infant pairs during the first three to four months of the babies’ lives in 2015. Participants were invited to attend regular check-ups at a local health clinic, where any cold- or flu-like symptoms were recorded and nasal swabs were collected. These samples were tested for a genetic sequence, IS841, which is found in B. pertussis, using a highly sensitive method called quantitative PCR (qPCR).. A stronger IS481 signal from the qPCR tests indicated that more bacteria were present in samples.
Crucially, the researchers did not rely on symptoms to trigger testing. Everyone was tested on a fixed schedule, regardless of how they felt – allowing the team to detect hidden infections that might otherwise have been missed.
To understand how infections may spread between mothers and infants, the researchers used statistical survival models. These models assessed whether a stronger IS481 signal in either the mother or the infant increased the risk of a subsequent positive test in their counterpart. This showed that if a mother tested positive – especially with a strong or intermediate signal – her infant was significantly more likely to test positive for the first time at the same or next visit. Even weak signals in mothers, which might typically be overlooked in clinical settings, predicted future infections in their babies. While positive signals in infants could also predict later detections in mothers, the effect was weaker and less consistent.
Next, the team looked at whether stronger IS481 signals were linked to symptoms. In infants, there was a clear correlation: stronger signals meant a higher likelihood of symptoms such as coughing or a runny nose. In mothers, by contrast, even strong signals were often not linked to symptoms – suggesting that adult infections are often silent or extremely mild.
The researchers then looked for persistent infections by searching for individuals in the cohort who had three or more positive IS481 tests during the study period. They identified 50 infants and 54 mothers who had signs of persistent infection, with a media duration of eight weeks. Within this group, many of the mothers showed no symptoms at all, further supporting the idea that adults can carry and potentially spread the disease for weeks without visible illness.
While the study offers important new insights into asymptomatic pertussis transmission, the authors acknowledge some limitations. The team relied on qPCR testing to detect pertussis, which is highly sensitive but can be affected by sampling variability. The authors also note that their study focused on one urban population over a single year, and that broader surveillance would be needed to confirm how common these patterns are in other regions and over time.
“Our findings show that a notable amount of pertussis transmission is probably under the radar,” says senior author Professor Pejman Rohani, a Professor at the Odum School of Ecology, University of Georgia. "We’ve also shown how effective surveillance can be done in low-resource settings using simple tools and local partnerships. Future work should focus on scaling up this approach and integrating it into national health systems to better understand and control the spread of this preventable disease.”
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