The top portion of the campus entrance gate showing IISER Pune logo

Persistent effects on the sense of smell after COVID-19 infection

Posted on
Share this article

In a new paper in the journal Current Research in Neurobiology, Dr. Nixon Abraham’s group from the Biology department at IISER Pune reported on persistent olfactory learning deficits during and post-COVID-19 infection. This work was carried out in collaboration with researchers from BJ Government Medical College and Sassoon General Hospitals, Pune.

Dr. Abraham’s group works on deciphering the neural circuits involved in olfaction, or the sense of smell, with the goal of obtaining insights into animal behavioural responses, decision-making processes, and brain functioning in health and disease. 

When loss or deficits in the sense of smell due to COVID-19 infection had just about begun to be reported, Dr. Abraham’s group chose to probe the matter and initiated a collaboration with doctors from BJ Government Medical College and Sassoon General Hospitals in Pune.

For experiments that involve the study of the sense of smell, the group employs olfactometer, a device to deliver odour stimuli in a controlled manner. Working through the early months of the COVID-19 pandemic in 2020, Dr. Abraham’s group custom-built a special version of the olfactometer with built-in safety precautions for preventing transfer of infection during the study and thus to be able to conduct an olfactory fitness study in COVID-19 patients. Their results, published in October 2020 in the journal eClinical Medicine, had identified deficits in the sense of the smell in over 80% of asymptomatic carriers.  

In the new paper published in March this year, the team conducted a high-precision quantitative assessment of the ability to smell in four groups of people: symptomatic COVID-19 patients, asymptomatic COVID-19 carriers, those who recovered from COVID-19 infection, and healthy individuals. 

Participants in the study were presented with certain odours at intervals and the following aspects were looked for and measured quantitatively by the team: did the participant detect the odour correctly, were they able to tell apart one odour from another (olfactory matching accuracy), and were there any changes to olfactory matching accuracy after multiple sessions (olfactory learning). 

The team then compared and analysed data they gathered via the olfactometer from each of these sessions, which together represented about 12,000 behavioural read-outs from a total of 200 participants. The team made the following observations.

Detectability indices and olfactory matching accuracies were significantly lower for symptomatic COVID-19 patients in comparison to healthy subjects. This observation was consistent for all concentrations tested using 10 different odorants (types of smells). 

Symptomatic patients and asymptomatic carriers showed differential odor detectability and olfactory function scores

Read-outs from COVID-19 patients and recovered individuals pointed to severe and persistent deficits in olfactory learning

“Such precise quantification of the sense of smell, where we were able to get information on both the sensory and cognitive deficits caused by COVID-19 infection, was made possible because of the olfactometer that we custom-built. Its design prevents cross-contamination between different tests, which is a unique feature, and also allows us to generate stimuli of varying complexity by using multiple odors,” said Dr. Nixon Abraham.

The team hopes to further develop this method to screen for neurodegenerative conditions and infections that affect the sense of smell, and to identify conditions such as hyposmia and anosmia at early stages. 

This work has received research grant support from the DST-Cognitive Science Research Initiative, Govt. of India and DBT/Wellcome Trust India Alliance.


Rajdeep Bhowmik, Meenakshi Pardasani, Sarang Mahajan, Rahul Magar, Samir V. Joshi, Ganesh Ashish Nair, Anindya S. Bhattacharjee, and Nixon M. Abraham (2023). Persistent olfactory learning deficits during and post-COVID-19 infection. Current Research in Neurobiology 4: 100081.

- With inputs from Dr. Abraham Nixon