Why mass vaccination campaigns count

Polio and measles are two deadly outbreak-prone childhood diseases preventable by vaccination. Rubella is less severe than the two but a leading cause of birth defects of the brain, heart and eyes in babies born to women who contact rubella in the early in pregnancy.

Polio is a serious disease that causes permanent paralysis in survivors. Measles causes severe life-threatening illnesses in children under five years and is a leading cause of blindness.

Polio and measles have potential for global eradication if population vaccination rates of greater than 95 percent are achieved and consistently maintained depriving circulating viruses a susceptible human host.

Routine immunisation in health facilities is the cornerstone strategy in protecting children against these vaccine preventable diseases. In most low-and middle-income countries (LMICs) including Kenya, high immunisation coverage has been difficult to sustain due to perennial disruptions in routine immunisation services from various causes such as industrial health worker actions.

These interruptions to health provision including immunisation, coupled with subnational inequities in coverage occasioned by highly mobile nomadic populations and refugees, vaccine hesitancy and compounded by the ongoing Covid-19 pandemic lead to growing clusters of unimmunised and under-immunised children at risk of contracting polio and measles which are known to be highly infectious.

Mass vaccination campaigns also known as supplementary immunisation activities (SIAs) are conducted periodically to close the immunity gaps. SIAs are conducted by vaccinating all children in the target age group, regardless of their vaccination status.

Immunisation activities are carried out simultaneously in the areas designated as high-risk for outbreaks as shown by surveillance data. Vaccines administered during these activities are supplemental and the child must still get the scheduled vaccines during routine immunisation clinics.

There is no harm in re-vaccinating a fully vaccinated child as extra vaccination acts as a booster. The usual target age group is the under-fives who are the highest risk age-group for most vaccine preventable diseases. A well conducted campaign ensures that all unimmunised, under-immunised and fully immunised children receive protection at the same time and this is the best wall of defence against circulating viruses.

Kenya has been wild poliovirus (WPV) free since 2014. However, the success of eliminating wild poliovirus has been curtailed by the detection of circulating Vaccine Derived Polioviruses (cVDPVs) in polio surveillance sites in recent years.

Mutations of virus strains contained in oral polio virus vaccines are responsible for the emergence of cVDPVs . These mutant strains have the potential to spread among under-immunised populations and may even lead to paralysis.

Outbreaks of cVDPVs can be halted by 2-3 rounds of polio SIAs aiming to achieve at least 90 percent coverage in the target at risk population.

In February 2021, six cases of cVDPV type 2 were detected in Garissa and Mombasa. This necessitated the ongoing polio SIA that will be conducted in two rounds spaced four weeks apart. The first round was in May 2021 and happened end of June.

Measles and rubella are the other vaccine-preventable diseases where high immunisation coverage rates greater than 90 percent are needed to ensure adequate population protection from outbreaks.

COVID DISRUPTION

Two doses of combined measles-rubella vaccine (MR) given at nine and 18 months are needed to achieve at least 95 percent protection. Only 85 percent of children vaccinated with one dose of MR vaccine obtain immunity against measles.

Over time, a cohort of unimmunised children accumulates leading to measles outbreaks. Routine immunisation services were disrupted in the early months of the Covid-19 pandemic, many parents delayed or missed MR vaccination out of fear of contracting or exposing their children to Covid-19.

The last MR-SIA was conducted in 2016. It was highly successful and achieved 95 percent coverage of the target population.

A successful campaign ensures that all children achieve protection against measles and serve the longer-term goal of steering Kenya towards elimination of measles and rubella.

Dr Christine Karanja-Chege, Lecturer, Department of Paediatrics, Kenyatta University; Collins Tabu, head, immunisation programme, Ministry of Health.