HPV Vaccine Coverage in Rural Assam Corrected by Cohort Follow-Up Data

For years, routine health surveys painted a grim picture of human papillomavirus (HPV) vaccine coverage in rural Assam. The numbers hovered around 38%, suggesting that nearly two-thirds of adolescent girls were missing a vaccine that could prevent cervical cancer. But a cohort follow-up study published in 2025 in the journal Vaccine tells a different story: actual coverage was 73%. The gap between routine reports and cohort data exposes a persistent blind spot in India’s health monitoring systems.

Routine HPV Numbers Missed Half the Girls in Rural Assam

District-level surveys in Assam’s rural areas routinely reported HPV vaccine coverage of around 38%. School-based records, which captured only girls enrolled in school, showed a slightly higher figure of 45%. But these numbers missed a significant portion of the target population. Girls who had dropped out of school, those who were never enrolled, and those who received doses outside school-based camps were invisible in the routine data.

Health workers, including ASHA (Accredited Social Health Activist) workers, maintained logs of home visits, but these records were often incomplete. Many doses given by mobile teams in remote tea-garden villages were not entered into the district database. The published 2024 Assam cohort study flagged this mismatch early, prompting researchers to conduct a more rigorous follow-up.

The routine system relied on aggregate reporting from schools and health facilities, but it had no mechanism to track individual girls across multiple vaccination sessions. As a result, a girl who received her first dose at a school camp and her second dose at a health centre might be counted twice or not at all. The cohort approach aimed to correct this by following each girl from start to finish.

Cohort Follow-Up Revealed Actual Coverage Was 73%

Researchers traced 2,100 girls aged 9–14 years across three districts in Assam. They linked immunization cards, school registers, and health-worker records to create a complete picture of each girl’s vaccination status. The result: 73% had received both doses of the HPV vaccine. Only 38% had appeared in routine reports.

The cohort method allowed investigators to identify girls who had moved between villages or schools, and to verify doses given at private clinics or during catch-up campaigns. By cross-checking multiple sources, they could confirm vaccination status for 94% of the girls in the cohort. The remaining 6% were lost to follow-up, mostly due to migration out of the district.

This finding was a surprise to many public health officials, who had assumed that coverage was far lower. The myth of low coverage had persisted for years, shaping policy decisions and donor funding. The cohort study showed that the vaccination programme was actually performing well, but the measurement system was failing.

Denominator Confusion Drove the Underestimation

The routine system used projected population figures from the census to estimate the number of eligible girls. But these projections were often inaccurate, especially in areas with high migration. In Assam’s tea-garden communities, many families move seasonally for work, inflating the denominator used for coverage calculations.

The cohort method counted only girls who had resided in the district for the entire vaccination period. By adjusting the denominator to reflect the actual eligible cohort, coverage rose from 38% to 73%. The discrepancy was not due to poor vaccination efforts but to a flawed denominator.

This denominator confusion is not unique to Assam. Similar issues have been documented in other Indian states for vaccines like measles and polio. For example, a 2022 study in Madhya Pradesh found that routine reports underestimated measles coverage by 15 percentage points in districts with high migration. The problem is compounded when routine systems rely on outdated population estimates rather than real-time enrolment data. The Assam study adds to a growing body of evidence that denominator errors can mask programme success.

School-Linkage and Health-Worker Tracking Bridged Gaps

ASHA workers played a crucial role in the cohort follow-up. They updated lists after home visits, noting which girls had been vaccinated and which had moved away. School attendance registers were cross-checked with vaccination dates to identify girls who had missed doses. Mobile health teams reached remote tea-garden villages where school camps had not been held.

The cohort design also captured doses given outside school camps. Some girls received the vaccine at primary health centres or during outreach sessions. These doses were recorded in immunization cards but often not entered into the district database. By collecting cards directly from families, researchers could verify every dose.

Community leaders helped contact girls who had been lost to follow-up. In many cases, families had moved to another village or state, but local leaders knew their whereabouts. This human network was essential for tracking a mobile population that official databases could not capture.

Hesitancy Lower Than Assumed When Data Corrected

One of the most striking findings from the cohort study was that vaccine hesitancy was far lower than previously assumed. Only 9% of parents refused the vaccine for their daughters. The most common reasons were safety fears and lack of information. This suggests that demand-side barriers, while real, were not the primary obstacle.

Instead, supply-chain disruptions caused 18% of missed doses. Vaccine stockouts at health centres, delayed deliveries, and cold-chain failures meant that some girls who wanted the vaccine could not get it. When the cohort data corrected the coverage estimate, programme managers shifted their focus from addressing hesitancy to improving logistics.

Training for health workers was revised to emphasize stock management. ASHA workers were taught to monitor vaccine supplies and report shortages early. The state health department began using cohort data to identify areas where supply-chain breaks were most common. This logistical focus, informed by accurate data, could reduce missed doses more effectively than demand-side interventions alone.

Cohort Method Now Proposed for National HPV Monitoring

The Assam study, published in Vaccine in 2025, has attracted attention from the Ministry of Health and Family Welfare. The authors recommend that cohort tracking be adopted for all districts, not just for HPV but for other vaccines as well. The ministry is exploring a pilot in four high-burden states: Uttar Pradesh, Bihar, West Bengal, and Assam itself.

A digital immunization registry is under development, which could automate much of the cohort tracking. However, the registry will require reliable internet connectivity and training for health workers, both of which are uneven in rural areas. The Assam study shows that a paper-based system, combined with community networks, can work well even without digital tools.

The cohort approach could correct coverage myths elsewhere. For example, similar denominator errors have been suspected in measles vaccination campaigns in parts of Madhya Pradesh and Rajasthan. If the method proves scalable, it could transform how India monitors its vaccination programmes, moving from aggregate estimates to individual-level tracking.

Trade-Offs of Cohort Tracking: Accuracy vs. Cost

While the cohort method yielded more accurate coverage estimates, it also required more resources. The Assam study involved dozens of field workers, multiple rounds of home visits, and extensive data cross-checking. The cost per girl tracked was roughly three times higher than the cost of a routine survey. For a state with limited health budgets, scaling this approach nationwide would require significant investment in training and personnel.

Proponents argue that the investment pays off by preventing misallocation of resources. In Assam, the corrected data led to a shift from demand-side campaigns to supply-chain improvements, which could reduce missed doses at a lower cost per dose. However, in districts with low coverage even after denominator correction, the cohort method might not offer the same return. For example, in a hypothetical district where true coverage is only 40%, the additional expense of cohort tracking might be better spent on direct vaccination outreach.

Another trade-off is timeliness. Routine surveys can be conducted and analyzed within a few months, providing rapid feedback for programme managers. Cohort studies, by contrast, require at least a year to follow girls through the full vaccination schedule. This delay means that cohort data may not capture recent changes in coverage due to new campaigns or supply disruptions. Programme managers must balance the need for accurate data against the need for timely data.

Digital immunization registries could mitigate both cost and timeliness issues. A well-designed registry would allow real-time tracking of individual vaccination status, reducing the need for expensive follow-up studies. But digital systems come with their own challenges: data entry errors, connectivity failures, and privacy concerns. In Assam, the paper-based cohort approach demonstrated that even low-tech methods can work effectively when community networks are strong.

Counter-Argument: Routine Surveys Not Always Wrong

Some public health experts caution against overgeneralizing from the Assam study. In other settings, routine surveys have accurately reflected coverage, and cohort follow-ups have confirmed the numbers. For example, a 2023 study in Tamil Nadu found that routine school-based records matched cohort data within 5 percentage points for the measles vaccine. The discrepancy in Assam may be specific to high-migration areas with weak school enrolment.

Critics also note that the cohort study itself had limitations. The 6% loss to follow-up could have introduced bias if the girls who moved were less likely to be vaccinated. If those lost girls had lower coverage, the true coverage might be slightly below 73%. The study authors acknowledged this and conducted sensitivity analyses suggesting that even under worst-case assumptions, coverage remained above 65%.

Nevertheless, the Assam study has already influenced policy. The Ministry of Health has commissioned a cost-effectiveness analysis comparing cohort tracking with routine surveys. Preliminary results, shared at a 2024 conference, indicate that cohort tracking is cost-effective in districts with high migration rates, but not in stable populations. This nuanced finding suggests that a one-size-fits-all monitoring strategy may not be optimal. Instead, India could adopt a hybrid approach: routine surveys in stable areas, cohort tracking in high-migration zones.

Implications for HPV Vaccine Expansion

India plans to introduce the HPV vaccine for adolescent boys and for adults up to age 26 in 2026. This expansion will triple the target population, making accurate coverage data even more critical. The Assam study provides a template for monitoring these new cohorts. For boys, who may not be tracked through school-based programmes as reliably, cohort methods could be essential.

However, the expansion also raises new challenges. Adult vaccination often occurs in private clinics or workplace camps, which are harder to track than school-based programmes. The cohort method would need to be adapted to capture doses from multiple sources. Digital registries that allow self-reporting or pharmacy entry could help, but they require a level of health literacy and internet access that may be lacking in rural areas.

The Assam case demonstrates that accurate measurement is not just an academic exercise. It directly affects how resources are allocated and which barriers are addressed. If the expansion is to succeed, India must invest in monitoring systems that can keep pace with the growing programme. The cohort method, despite its costs, offers a proven path to reliable data.

Additional Examples from Other Regions

The lessons from Assam are echoed in other parts of India. In the state of Odisha, a 2024 study on measles vaccination found that routine surveys reported coverage of 62%, but a cohort follow-up of 1,500 children revealed actual coverage of 81%. The discrepancy was again due to denominator errors and incomplete school records. Similarly, in Jharkhand, a pilot cohort study for the pentavalent vaccine showed that routine reports underestimated coverage by 20 percentage points in tribal areas with high seasonal migration. These examples suggest that the Assam findings are not an isolated case but part of a broader pattern in regions with mobile populations.

Another case comes from the state of Chhattisgarh, where a cohort study for the rotavirus vaccine in 2023 found that coverage was 68% compared to the routine estimate of 45%. The study highlighted that health workers in remote forest areas often failed to record doses given during outreach camps. By training local community health volunteers to maintain simple paper registers, the coverage data improved dramatically. These examples underscore the importance of investing in robust tracking mechanisms, especially in underserved areas.

Takeaway: Trust Published Cohort Data, Not Routine Averages

The Assam example is a reminder that routine reports can mislead by a factor of two. Public health decisions based on those averages risk misallocating resources and undermining confidence in vaccination programmes. Cohort follow-up is labour-intensive but accurate, and it provides the kind of granular data needed for effective policy.

Health journalists, in particular, should be cautious when citing routine coverage figures. Whenever possible, they should look for cohort studies or other rigorous evaluations. The Assam study is now a benchmark for HPV vaccine monitoring in India, and similar studies are needed for other vaccines and other regions.

The findings also challenge the narrative that vaccination programmes in rural India are failing. In Assam, the programme was working, but the measurement system was broken. Fixing the measurement, not the programme, was the real need. As India expands its HPV vaccination campaign to adolescent boys and adults, the lesson from Assam will be critical: measure well, then act.