Do children need another measles vaccine if they have already received it?
Extra dose helps close immunity gaps in children, Child specialist Dr Abu Sayeed Shimul says
Bangladesh is currently conducting a nationwide measles-rubella (MR) vaccination campaign amid a surge in measles infections.
Many parents ask: if a child has already received the routine two doses, why is an additional dose needed?
Child specialist Dr Abu Sayeed Shimul of Mugda Medical College Hospital says the additional dose is needed to close immunity gaps, as not all vaccinated children develop sufficient protection to prevent transmission.
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Measles and herd immunity
He explains that measles is one of the most infectious diseases, with a single infected person capable of transmitting the virus to 12-18 susceptible individuals.
Because of this high transmissibility, the herd immunity threshold required to stop measles is more than 95%.
Herd immunity means that when a large proportion of the population is immune, disease transmission is interrupted, protecting the entire community.
Not all infectious diseases require such a high level of immunity, he says. For example:
- Influenza: around 50–70%
- Covid-19 (early estimates): around 60–70%
- Measles: highest at around 92–95% or more
Immunity gap despite vaccination
Although two doses of the measles vaccine provide about 97% protection, a small proportion of children remain unprotected.
In densely populated countries like Bangladesh, even a small immunity gap can sustain outbreaks, Dr Shimul says.
Why the additional dose is needed
He explains that herd immunity depends on how many people are effectively immune, not just how many have received vaccines.
Even after one or two doses, some children fail to develop adequate immunity, leaving them susceptible.
As a result, even if 95% of children are vaccinated, the number of those actually immune may fall below the level required to stop transmission.
How the extra dose works
The additional dose during campaigns helps close this immunity gap.
Dr Shimul explains its role as follows:
- It helps protect children who did not develop immunity earlier
- It boosts immunity in those with weaker protection
- It increases the number of effectively immune individuals
This helps raise overall immunity to the level needed to interrupt measles transmission.
Why all children are vaccinated
He says it is not feasible in field settings to identify which children are immune and which are not.
For this reason, the World Health Organization recommends vaccinating all eligible children during campaigns, regardless of prior vaccination status.
Role of mass campaigns
Supplementary immunisation activities are critical for rapidly increasing population immunity and closing immunity gaps, he says.
These campaigns help achieve a uniform level of protection across the population, which is necessary to break transmission chains.
Key message
Dr Shimul says the extra MR dose in campaigns is not mandatory for individual protection but is strongly recommended for outbreak control.
The dose is safe and beneficial even for children who have already received two prior doses.
He adds that achieving effective immunity of at least 95% is essential to control measles.
Vaccination campaign
A month-long measles vaccination campaign is set to begin tomorrow (12 April) across four major city corporations, targeting nearly 12 lakh children, health officials said.
The campaign will roll out in Dhaka North, Dhaka South, Mymensingh and Barishal city corporations from 12 April to 11 May.
Vaccinations will be administered daily from 8am to 4pm, excluding public holidays. A nationwide rollout is scheduled to begin on 20 April.
