At 11 on a Thursday morning, Rajkumari balances a blue vaccine cold box in one hand and a pink file in the other as she knocks on an iron gate in Vijay Nagar, a densely packed neighbourhood in Ghaziabad.Rajkumari asks the baby’s name, age and father’s name, recording each answer in her register. Beside her, an ASHA worker gently tilts the infant’s head back. Two drops of polio vaccine land on her tongue. The baby bursts into tears.Before leaving, Rajkumari pauses.“No child has been missed, right?” she asks. “Tell me honestly.”The team moves on.They weave through narrow lanes lined with closely packed houses. Some doors stay shut despite repeated knocks. Others are locked, their occupants away at work. Each missed house is marked in Rajkumari’s register for another visit.A few hundred metres away, Vijay Nagar’s concrete homes give way to a cluster of tin-roofed shelters beneath an under-construction apartment tower. Families from Bihar and nearby districts of Uttar Pradesh have made homes here, many of the men working at construction sites while the women care for children or work as domestic helpers.Story continues below this adStepping past piles of garbage and algae-covered drains, Rajkumari and Manisha stop at every shack.“Is there a child under five?”When Shyam, a 32-year-old carpenter from Darbhanga, spots the health workers, he calls after his young son before the boy disappears into the maze of homes.“Isko bhi pila do (Give him the drops too),” he says, scooping him into his arms.Shyam says vaccination campaigns were a familiar part of life in his village.Story continues below this ad“I moved here four years ago, but the anganwadi workers would set up booths at government schools back home,” he says. “Many people are afraid. They don’t know what these drops are for. But I know they’re important.”Only later does the reason for Rajkumari’s persistence become clear.Every household now gets the same question more than once. If someone says there are no children inside, she asks again before moving on.The reason lies in a sewage sample collected from Vijay Nagar on June 5. It tested positive for vaccine-derived poliovirus, a rare strain that can circulate where immunisation coverage is low.Story continues below this adIndia was declared free of wild poliovirus in 2014 after decades of mass immunisation. But environmental surveillance – testing sewage for traces of the virus – continues as an early warning system. After the detection in Ghaziabad, officials traced the sewage network, identified 12 neighbourhoods home to about 150,000 people as high-risk areas, and intensified vaccination efforts under the watch of the National Centre for Disease Control and the World Health Organisation.Rajkumari is one of hundreds of health workers now working under a far more intensive campaign.After the sewage detection, officials strengthened supervision, redrew vaccination maps and instructed teams to revisit locked homes until every eligible child had been reached.“We reviewed every area covered in the last round to make sure nothing had been missed,” says Dr Soniya Patel, the Medical Officer In-charge overseeing the campaign. “We cannot miss any child.”Story continues below this adBut reluctance is not confined to the city’s poorest neighbourhoods.Amit Pal, 22, another vaccination worker, says some parents refuse to open their doors or insist there are no young children inside. If persuasion fails, supervisors return to counsel the family.“We cannot leave out any child,” he says.Across the road, in the gated towers of Paramount Symphony, the conversations are different, but the hesitation can be much the same.Auxiliary Nurse Midwife Usha Yadav recently spent 45 minutes persuading a mother to allow her child to receive the vaccine. Only after showing news reports on her phone about the virus detected in Ghaziabad’s sewage did the woman agree.“Only then did she agree,” Yadav says.Story continues below this adWhether the doors belong to tin-roofed shacks or high-rise apartments, the challenge is the same: convince every parent, reach every child.By 1 pm., Rajkumari returns to the Ayushman Arogya Mandir after five hours on foot. Her register is filled with names, but also with houses to revisit.Some families were away. Some needed persuading. Others will be visited again over the next few days.Before turning away from each door, she asks the same question.Story continues below this ad“Chuth toh nahi gaya koi baccha? (No child has been missed?)