Mathematical modelling of infectious diseases is not just an academic exercise but also provides data driven evidence that could influence real-world policy and public health decisions based on how a disease affects a population.

Sandip Mandal, our next pathbreaker, Independent Consultant with several UN organisations and international agencies, primarily focuses on infectious disease modelling, particularly tuberculosis in order to understand how the disease spreads, evaluating interventions and identifying strategies that can have the greatest impact on public health.

Sandip talks to Shyam Krishnamurthy from The Interview Portal about his work that helps policymakers identify effective strategies to prevent and control infectious diseases. Mathematical models help evaluate different public health approaches that can improve health policies and ultimately help prevent disease and save lives.

For students, whatever you pursue, the ability to observe carefully, think logically, and imagine creatively should be nurtured from an early stage of life. These skills are equally essential whether one is creating a painting or solving a problem in science.

Sandip, Your background?

I grew up in Sainthia, a small town in the Birbhum district of West Bengal, where my father ran a grocery shop and my mother was a homemaker. Although we came from a modest background, education was highly valued in our family. My father, who had a degree in literature, was an avid reader and regularly shared with me ideas and discoveries from subjects ranging from history and literature to modern science, which sparked my curiosity and love for learning. My mother, whose education ended after Grade 8 because opportunities for girls in her remote village were limited, always encouraged me to pursue my studies with determination, hoping that I would secure a government job, a common aspiration for lower middle-class families in our town.

From an early age, I loved reading books beyond my school curriculum and developed a strong interest in drawing and painting. In fact, throughout my childhood and until Grade 12, my dream was to become an artist. However, after performing well in my Grade 10 and 12 examinations and recognizing the uncertainty of a career in the arts, I chose to pursue a degree in Physics, a subject I enjoyed the most among my academic disciplines.

I believe that whatever you pursue, the ability to observe carefully, think logically, and imagine creatively should be nurtured from an early stage of life. These skills are equally essential whether one is creating a painting or solving a problem in mathematics or physics.

What did you do for graduation/post graduation?

I completed my BSc in Physics at Abhedananda Mahavidyalaya, a college in my hometown, and went on to earn my MSc in Physics from Visva-Bharati University in Santiniketan, West Bengal. I later completed my PhD at Visva-Bharati University, specializing in mathematical biology.

However, this journey was far from straightforward. My academic and professional path was marked by numerous challenges, setbacks, and turning points that shaped my career.

What were some of the influences that led you to such an offbeat, unconventional and uncommon career in Public Health?

My career has been shaped more by chance than by a carefully planned choice. Growing up, I was not the kind of student who consistently topped school or college examinations. However, I always believed that talent alone is not enough. Whatever I undertook, I gave it my full effort and perseverance, and I learned never to let setbacks determine my future.

Several people and ideas have influenced my journey. At a philosophical level, the person who has shaped my worldview the most is Rabindranath Tagore. His writings on humanity, education, and universalism have had a lasting impact on how I think about life and society. They taught me to value curiosity, compassion, and the pursuit of knowledge beyond narrow boundaries.

From a career perspective, one of my earliest inspirations was Dr. Ananda Mohan Chakrabarty, a renowned scientist from my hometown. I came to know about him in my school days. His pioneering work on the genetically engineered Pseudomonas bacterium capable of breaking down crude oil, and his landmark achievement of obtaining the first patent on a genetically modified living organism, showed me that someone from a small town could make discoveries with global impact. Physically I could never meet him.

Later, I saw the name of our own biology teacher Dr Somnath Bhakat in a local newspaper mentioning that he got an invitation from France to present his research work. His success broadened my horizons and inspired me to believe that scientific research could be a meaningful and impactful career.

After completing my master’s degree at Visva-Bharati University, I joined IIT Kharagpur to pursue a PhD in experimental physics, focusing on nonlinear optics. However, due to some unexpected difficulties in my relationship with my supervisor—a situation that, unfortunately, is not uncommon in India—I decided to leave IIT Kharagpur. By then, I had lost almost two years of my career there. I subsequently joined a government school in a village in West Bengal as a schoolteacher.

At that point, I decided to return to my university and continue my PhD as a part-time scholar. Interestingly, the knowledge of computer programming that I had acquired during my coursework at IIT Kharagpur opened up an entirely new direction for me. It enabled me to explore mathematical biology and eventually shaped much of my subsequent research career. Looking back, therefore, I do not consider my time at IIT Kharagpur a waste at all. In fact, it helped me in two important ways. First, it gave me the confidence to think big and explore areas beyond my immediate academic background. Second, it provided me with technical skills in computer programming at a time when I had hardly any experience with computers—I had barely known how to switch one on.

At Visva-Bharati, my PhD supervisor, Prof. Samar K. Roy, was not very familiar with the specific work I was doing, as it was outside his area of expertise. However, he was extremely open-minded and supportive. Rather than restricting me to his own field, he introduced me to several people who could guide and help me in different ways. Through him, I was introduced to my joint supervisor, Prof. Santanu Ray of the Department of Zoology, as well as Dr. Madan Mohan Panja, who later became a Professor of Mathematics at Visva-Bharati University.

In the physical world, every object and phenomenon follows certain underlying rules. As scientists, we try to uncover these rules and understand how and why things happen. Mathematics provides a powerful language for expressing these rules through equations. Mathematical biology applies the same approach to living systems, using mathematical equations to describe, understand, and predict biological phenomena.

In my PhD thesis, I studied how aquatic ecosystems maintain their stability under different conditions and how far they can adapt or self-organise before they eventually collapse. For example, in an aquatic ecosystem, phytoplankton are grazed by zooplankton, which are then eaten by small fish. These interactions and the dependencies among different species can be represented using mathematical equations, with parameters such as grazing rates, birth rates and death rates.

These equations are often too complex to solve analytically, so we use computer algorithms to obtain numerical solutions. By changing the parameters within realistic range, we can simulate different conditions and examine how the ecosystem responds—for example, whether it remains stable, changes to a new state, or collapses.

My PhD work was primarily aimed at developing a theoretical understanding of ecosystem dynamics rather than addressing a direct real-world problem. However, the same mathematical modelling approach has very practical applications. Later in this interview, I’ll explain how we use mathematical biology to understand disease dynamics and address real-world public health problems.

Prof. Panja’s dedication to his work had a profound influence on me. His commitment and enthusiasm inspired me greatly and strengthened my determination to pursue research as a career. By then, I had become very clear about my future: after completing my PhD, I wanted to undertake postdoctoral research and build my career as a researcher.  

Looking back, there was no single turning point that determined my career. Rather, it was a combination of perseverance, the influence of inspiring role models, and the willingness to embrace unexpected opportunities that gradually led me to where I am today.

How did you plan the steps to get into the career you wanted?

After submitting my PhD thesis, I got an opportunity to do postdoctoral research at the CSIR–Centre for Cellular and Molecular Biology (CCMB) in Hyderabad. I left my teaching job and began working in the field of disease modelling, trying to understand how infectious diseases spread and how mathematical models could help us identify effective ways to control them. At CCMB, I learned a great deal from Prof. Somdatta Sinha, particularly how to look at data not just from a theoretical perspective, but also how to use data to address practical and real-world problems.

After spending three years at CCMB, I got another opportunity to go to Tokyo, Japan, as a postdoctoral fellow. My project there involved developing mathematical models of ocean ecosystems incorporating ocean currents, with the aim of understanding phytoplankton dynamics. Although this was a completely different field from infectious disease modelling, the fundamental principles of mathematical modelling that I had learned at CCMB and Visva-Bharati proved to be highly transferable. They allowed me to understand the new problem and develop the model relatively quickly.

At that stage, I had not yet decided to pursue a career specifically in disease modelling. I was primarily interested in mathematical biology and wanted to gain international research exposure. The opportunity at Tokyo was particularly valuable for two reasons.

First, I had the opportunity to participate directly in data collection and work closely with the data. This was an important experience because, during my earlier work at Visva-Bharati and CCMB, I had mainly worked with existing datasets and had less exposure to the process of collecting and understanding data at source.

Second, the model we developed at Tokyo went beyond a purely biological model. We incorporated ocean currents into the biological model to understand how physical processes in the marine environment influence biological dynamics. This gave me valuable experience in integrating different types of processes into a mathematical model and helped broaden my understanding of how mathematical modelling can be applied to real-world systems. 

My supervisor in Tokyo, Prof. Yamazaki, was very impressed with my work. Even after I returned to India, he continued to collaborate with me as a consultant on some of his projects. This experience reinforced something I had gradually begun to understand: the subject matter may change, but the underlying principles of mathematical modelling remain remarkably similar.

After returning to India, I joined the Public Health Foundation of India (PHFI) in Delhi as a Research Scientist. I was involved in a project to develop a mathematical model of tuberculosis in India, with the aim of using modelling to inform public health policy and decision-making. This was the beginning of the path that eventually led me to where I am today.

Incidentally, I had always wanted to settle in India because my parents were living here. So, although I had opportunities to work abroad, returning to India was always important to me.

At PHFI, I had the opportunity to work closely with Dr. Nimalan Arinaminpathy, who was then a young Assistant Professor at Imperial College London and is now at WHO in Geneva. In many ways, what I am doing today grew out of what I learned from him during that period. Looking back at my career, I feel particularly fortunate in one respect: whenever I entered a new phase of my professional life, I somehow found myself working with exceptional mentors and supervisors. Each of them taught me something different, and together they shaped the researcher I eventually became.

How did you get your first break?

My first major break came in early 2015, when our first paper on tuberculosis interventions was published in an Oxford University Press journal. Soon after the publication, I received a call from the WHO South-East Asia Regional Office in Delhi. Dr Swarup Sarkar, then Director of the Communicable Diseases Division, approached me with an ambitious question: what strategies would be needed to achieve the End TB goals across the eleven countries of the South-East Asia Region?

That became my first research project with WHO and, importantly, my first opportunity to use mathematical modelling to directly inform health policy. We successfully completed the project, and the findings contributed to discussions among the Health Ministers of the WHO South-East Asia Region. This ultimately led to the “Delhi Call for Action” to End TB, through which the countries committed to ending TB in the region by 2030.

For me, that was a turning point. It showed me that mathematical modelling was not just an academic exercise—it could provide evidence that could influence real-world policy and public health decisions.

Since then, I have had the opportunity to work on a wide range of projects with WHO and other international organisations, including the Stop TB Partnership, the World Bank and the Asian Development Bank.

In mid-2019, I joined the Indian Council of Medical Research as a Consultant Scientist. When the COVID-19 pandemic began, I became closely involved with the Government of India’s modelling efforts. During the pandemic, I worked on a range of questions, from quarantine and containment strategies to vaccination and preparedness for successive waves of COVID-19. Many of these modelling studies were undertaken in close collaboration with policymakers and contributed to public health decision-making during a very challenging period.

After the pandemic, the Government of India took on another ambitious task: estimating the tuberculosis burden not only at the national level but also for individual states. I led the modelling component of this work. On World TB Day in 2023, at an event in Varanasi, the Prime Minister announced that India had become the first country in the world to estimate its TB burden at the country level using its own mathematical modelling system.

That was one of the proudest moments of my professional life. It was particularly meaningful because the work had evolved from something that started as a mathematical modelling exercise into a national system for informing TB policy. Since then, I have been involved in estimating India’s TB burden every year, and the work has also been acknowledged in the WHO Global Tuberculosis Report.

Looking back, I would say that my first break came when WHO gave me the opportunity to demonstrate that mathematical modelling could be translated into practical policy. That opportunity opened the door to many subsequent collaborations and shaped the direction of my career.

What were some of the challenges you faced? How did you address them?

The challenges I have faced have been of different kinds. Some were personal and family-related, while others arose from my professional work and from trying to translate scientific evidence into policy.

Challenge 1: Balancing family responsibilities and my career

One of the most difficult periods of my life came soon after I completed my PhD and started my first postdoctoral position at the CSIR–Centre for Cellular and Molecular Biology (CCMB), Hyderabad. Around that time, my father-in-law passed away, and my son was born. My sister-in-law was still in college, and my mother-in-law was dependent on us. Under those circumstances, I decided that my wife should stay with her family to support them, while I continued with my research career.

As a result, I spent almost six years away from my family, moving between different places for research opportunities. At the same time, there was considerable uncertainty about my career because I was approaching the upper age limit for many government positions. There were periods when I was unsure about where my career would lead and whether I would get a stable position.

There was no quick solution to this situation. I learned to remain patient, continue working hard and believe that better opportunities would eventually come. Looking back, that period taught me resilience and the ability to continue moving forward even when the outcome was uncertain.

Challenge 2: Translating research into policy

Professionally, I have always enjoyed research, so I rarely experienced the scientific work itself as a burden. The greater challenge has been translating scientific findings into actual policy.

Mathematical models can provide evidence about what a health policy is likely to achieve, but policymakers have to consider many other factors—political, economic, social and operational. When working with government organisations, there can sometimes be difficult situations where scientific recommendations and other considerations do not fully align.

For example, during the COVID-19 pandemic, some of these tensions were widely discussed in the media, including in The New York Times article “As India’s Lethal Covid Wave Neared, Politics Overrode Science” published in 2021.

In such situations, I remind myself of the ultimate purpose of my work: to use evidence to help save lives. As an individual researcher, I cannot control political decisions, nor is it productive to simply confront the system. What I can do is present the scientific evidence as clearly and objectively as possible and explain what the mathematical models suggest about different policy choices.

Challenge 3: Finding a constructive way forward

Over the years, I have learned that influencing policy is not about insisting that policymakers accept everything that a model suggests. It is about building trust, communicating evidence clearly and understanding the practical constraints under which decisions have to be made.

When the evidence is strong, I try to explain the implications of the modelling, the uncertainties involved, and the potential consequences of different choices. In many cases, policymakers are receptive and adopt an appropriate and feasible strategy.

So, rather than seeing difficult situations as obstacles, I have learned to remain focused on the larger objective, communicate evidence patiently and look for practical solutions. This approach has helped me navigate both the personal uncertainties of my career and the professional challenges of working at the interface between science and policy.

Where do you work now?

In December 2023, at the age of 47, I received an appointment letter for a Senior Scientist position (Scientist F) at the ICMR headquarters. However, I realised that joining as a government employee could place restrictions on my ability to continue my international collaborations and independent work. After careful consideration, I decided to remain independent.

Currently, I work as an independent consultant with several UN organisations and international agencies. I also spend a proportion of my time working with Avenir Health, a US-based organisation. They offered me a position in the United States, but I negotiated an arrangement that allowed me to work for them from India. So, essentially, I work from home, which gives me considerable flexibility to contribute to multiple projects.

More recently, I also became a co-founder of EviMod Research, an organisation focused on evidence-based modelling for public health policy.

What problems do you solve?

My work primarily focuses on infectious disease modelling, particularly tuberculosis. I use mathematical and computational models to address policy-relevant questions for different countries around the world—for example, estimating disease burden, understanding how diseases spread, evaluating interventions and identifying strategies that can have the greatest impact on public health.

Although TB is my main area of expertise, I also work on other infectious diseases when there is an opportunity to apply modelling to an important public health question.

Mathematical modelling of infectious diseases means using mathematics to represent how a disease spreads through a population. For example, when an infected person interacts with uninfected people, there is a chance of disease transmission. The number of new infections depends on factors such as the number of infected and susceptible people, transmission rate, recovery rate, and contact patterns etc. These relationships can be expressed through mathematical equations.

For example, an infectious disease model can show how an outbreak may evolve under different scenarios. We can ask: what happens if we vaccinate 50% of the population, detect cases earlier, or reduce transmission? The power of modelling is that it allows us to test these scenarios mathematically and estimate their potential impact before implementing them in the real world.

I can give a few real-life examples from COVID-19 in India. In early 2020, when the pandemic was just beginning, we modelled how quarantining symptomatic individuals could reduce transmission. The findings supported advocacy for quarantine policies. Similarly, modelling helped address policy questions such as whom to vaccinate first and where to prioritise vaccination when vaccines were initially in short supply. In this way, mathematical models can provide evidence to inform public health policy decisions.

What skills are needed for this job, and how did you acquire them?

This type of work requires a combination of skills: mathematical modelling, computer programming, statistics, data analysis and, most importantly, an understanding of disease epidemiology and public health.

Interestingly, I did not come from a medical, statistics or computer science background. I have a Masters degree in Physics. I acquired most of these skills gradually through my research career—by learning programming, working with data, studying epidemiology and developing mathematical models to answer real-world health questions.

I believe one of the strengths of my career has been that I learned these skills because I needed them to solve problems. I did not learn programming or statistics simply as academic subjects; I learned them as tools for answering questions that mattered.

What is a typical day like?

One of the main reasons I chose not to take another time-bound government or private-sector position is that I value the freedom to organise my own time.

I work primarily from home, using a computer and working with data, models and scientific literature. There is rarely a completely typical day. Depending on the project, I might spend the day developing a mathematical model, analysing data, writing a scientific paper, preparing a report, attending discussions with international collaborators or thinking about a policy question.

I am generally busy with something throughout the day. It could be science, writing, teaching, or even painting. I have always disliked wasting time, so I try to use my time as productively as possible.

What do you love about this job?

There are several things I love about my work.

First, I can do most of my research with a computer and data. I don’t need a laboratory or large infrastructure to pursue an important research question. That gives me enormous independence.

Second, I have the flexibility to organise my day according to what needs to be done. I can work on multiple projects and collaborate with people from different countries and organisations.

But the most important thing is the impact. I find it incredibly rewarding when something that started as a mathematical model or a piece of analysis eventually contributes to a national or international public health policy.

For me, that is the real satisfaction of being a scientist—not simply publishing a paper, but seeing scientific evidence translated into decisions that can ultimately improve people’s lives.

How does your work benefit society?

I think I have partly addressed this point earlier, but the central purpose of my work is to help improve public health policy.

My work helps policymakers identify effective strategies to prevent and control infectious diseases. By using mathematical modelling to evaluate different public health interventions, I provide evidence that can improve health policies and ultimately help prevent disease and save lives.

Tell us an example of a specific memorable work you did that is very close to you!

This is difficult to answer because there is not just one. Several moments in my career have been particularly meaningful to me.

My first major project with WHO is one of them. The modelling work contributed to the “Delhi Call for Action” to End TB, when the Health Ministers of the eleven countries in the WHO South-East Asia Region came together and committed to ending TB.

Another memorable moment was during COVID-19, when the Indian Health Secretary, Mr Rajesh Bhushan, and the DG of ICMR, Dr Balram Bhargava, circulated our COVID-19 preparedness web tool to states across India for planning and preparedness.

Finally, the Prime Minister’s announcement that India had developed its own in-country mathematical modelling system for estimating TB burden, and the subsequent acknowledgement of this work in the WHO Global TB Report, were among the proudest moments of my career.

These experiences are special to me because they showed that my research could move beyond academic publications and actually contribute to national and international public health policy.

Your advice to students based on your experience?

My advice to students is simple: don’t focus too much on your career or earning money at an early stage of life. Instead, focus on learning, exploring different subjects and finding something that you genuinely enjoy doing.

Don’t take life too seriously. Enjoy the journey, keep learning and don’t worry too much about what others think or say. Everyone has a different journey, and sometimes it takes time to find your own path.

If you keep learning and doing what you love, eventually you will find your own path in life.

Future Plans?

My first goal is to see EViMod Research grow into an organisation that uses mathematical modelling and evidence to support health policy for a range of diseases, both in India and internationally.

My second goal is to build and mentor a group of young researchers who can carry this fascinating work forward. I have already started working towards this by being involved in various ICMR training programmes, as well as training initiatives at AIIMS Nagpur and IIT Bombay.

Ultimately, I would like to create not just an organisation, but a community of young researchers who can use modelling to solve real-world public health problems.