Nipah Virus: NIV Researchers’ Assertion Of Aerosol Transmission Not Backed By Evidence
While Acute Respiratory Distress Syndrome (ARDS) patients with cough and respiratory distress are very likely to expel Nipah virus into the air, there is no hard evidence to suggest that patients generate “virus-laden aerosols that can travel greater distances and remain suspended in the air for extended periods” and infect others
Since 2018, Kerala has reported a total of two Nipah outbreaks, in the years 2018 and 2023, involving human-to-human transmission, and seven spillovers from bats to humans in the years 2019 and 2021, and two events in 2024 and 2025 where only a single case was reported with no human-to-human transmission. So, what makes human-to-human transmission of Nipah virus possible leading to an outbreak and what ends as a spillover from bats to humans without spreading further among people, including close contacts?
Based on the 2001 outbreak in Siliguri, West Bengal, the 2007 outbreak in Nadia, West Bengal, the 2018 outbreak in Kozhikode, Kerala, and the 2023 outbreak again in Kozhikode, Kerala, researchers from the Pune-based National Institute of Virology (ICMR-NIV) underscore the “heightened transmissibility associated with acute respiratory distress syndrome (ARDS) manifestations”.
Nipah infection can manifest clinically as either acute encephalitis syndrome (AES), acute respiratory distress syndrome (ARDS), or a combination of AES and ARDS. While patients with AES do not have any respiratory distress and hence do not cough, patients with only ARDS or a combination of AES and ARDS have characteristic coughing and other respiratory symptoms. Patients with ARDS are more likely to spread the virus to others.
The primary case in the 2018 and 2023 outbreaks in Kerala had ARDS, and the virus spread to 20 and five contacts, respectively. Based on epidemiological studies, a 2019 paper in the Indian Journal of Community Medicine by doctors from the Department of Community Medicine, Government Medical College, Kozhikode found that the 2018 human-to-human transmission led to the primary case spreading the virus to 15 others. All the 15 secondary cases (two family members, two healthcare workers and 11 hospital visitors and patients) were exposed to the primary case during his stay in two healthcare settings. During the 2023 outbreak, the primary case showed signs “suggestive of ARDS” and spread the infection to two family members and three healthcare workers.
Based on the fact that ARDS patients are more likely to expel the virus while coughing, the NIV researchers make a sweeping statement in a July 2025 paper in the Journal of Infection and Public Health. “The characteristic coughing and other respiratory symptoms in ARDS patients generate virus-laden aerosols, which, due to their smaller particle size, can travel greater distances and remain suspended in the air for extended periods, increasing the risk of inhalation and subsequent infection,” they write.
“Most of the secondary cases were infected in hospital settings. The healthcare settings are marked by a closed environment with poor ventilation,” says Dr. Shubin Chenayil from the State Surveillance Unit, Directorate of Health Services, Malappuram, Kerala and a coauthor of the July 2025 paper that proposes aerosol transmission of Nipah virus from ARDS patients. “It is hard to say that the virus spreads through aerosols and not as droplets. But aerosol transmission in such settings is possible.”
Jumping to conclusion
While ARDS patients with cough and respiratory distress are very likely to expel Nipah virus into the air, there is no hard evidence to suggest that patients generate virus-laden aerosols that can “travel greater distances and remain suspended in the air for extended periods” and infect others. The July 2025 paper by the NIV researchers have ruled out droplet transmission and have instead concluded aerosol transmission not based on samples collected from distant places at different durations and studying the viability of the virus to survive as aerosols at different temperature and humidity conditions but merely based on the fact that ARDS patients with respiratory distress are very likely to expel the virus during coughing.
According to the WHO, airborne transmission refers to the presence of microbes within a droplet nuclei, which are generally considered to be particles less than 5μm in diameter, can remain in the air for long periods of time and be transmitted to others over distances greater than one metre. On the other hand, droplet transmission occurs when a person is in in close contact (within one metre) with someone who has respiratory symptoms (e.g., coughing or sneezing) and is therefore at risk of having his/her mucosae (mouth and nose) or conjunctiva (eyes) exposed to potentially infective respiratory droplets.
According to the 2019 paper, all 15 secondary cases were exposed to the primary case during his stay at the two hospitals and were infected during the late stages of illness of the primary case. If the virus is spread through aerosols as the NIV researches claim in the July 2025 paper, then there should have been many cases among the contacts. However, of the 2,642 contacts screened during the 2018 outbreak, only 23 cases (18 lab-confirmed) were found in 2018. Similarly, during the 2023 Nipah outbreak, of the 1,288 contacts, only six cases, including the index case, were infected.
Why aerosol transmission is unlikely
“Both during 2018 and 2023 outbreaks, there has been only limited secondary transmission despite close contact in healthcare and household settings,” says Dr. Anoop Kumar A.S., Director of critical care medicine at Aster MIMS, General Hospital, Kozhikode, Kerala, and who played a pivotal role in detecting the Nipah virus outbreak in the State in 2018. “Analysis of transmission patterns reveals that person-to-person transmission occurred primarily through direct contact.”
Dr. Kumar also says that the pattern of transmission during 2018 and 2023 outbreaks is consistent with contact and large droplet spread rather than airborne transmission, as infections were concentrated among those providing direct care rather than affecting individuals in adjacent rooms or areas. “No documented cases show transmission to individuals separated by significant distances from infected patients, which would be expected with true airborne transmission,” he says.
Dr. Kumar also says that research has demonstrated that Nipah virus has limited environmental stability under conditions that would support airborne spread. “Nipah virus survives less than 15 minutes at 37 degree C when dried, with a half-life of only 1.2-1.45 minutes at room temperature on non-porous surfaces,” he says. “While Nipah virus can survive in fruit juices and date palm sap for several days, this occurs in liquid media, not in aerosol particles.”
“The success of standard and droplet precautions in controlling Nipah outbreaks provides further evidence against airborne transmission. If Nipah virus is truly airborne, standard droplet precautions would be insufficient to prevent transmission,” he says.
Evidence from past outbreaks
Dr. Kumar cites earlier Nipah outbreaks in Malaysia, Bangladesh and India to question aerosol transmission of Nipah virus. A cohort study of 338 health-care workers who were exposed to the virus and 288 unexposed health-care workers during the 1998-199 outbreak in Malaysia found that despite 39 (11%) health-care workers had mucosal surface exposure to body fluids, and 89 (25%) health-care workers had skin exposure to body fluids, none were infected. “The risk of nosocomial transmission of Nipah virus appears to be low,” the authors write.
Multiple studies from Bangladesh hospitals treating Nipah patients demonstrated no evidence of infection among healthcare workers despite substantial exposures and minimal protective equipment use, Dr. Kumar says. For instance, a 2007 paper studying the risk of nosocomial transmission of Nipah virus in a Bangladesh hospital found “no evidence of Nipah virus infection despite substantial exposures and minimal use of personal protective equipment”.
“Even the Siliguri outbreak in 2001 showed that while 75% of cases occurred among hospital staff or visitors, transmission patterns were consistent with contact and large droplet spread rather than airborne transmission, as infections were concentrated among those providing direct patient care,” Dr. Kumar says.
