The 2013 measles outbreak in Sri Lanka: experience from a rural district and implications for measles elimination goals

In this study, we investigated a part of Sri Lanka’s most recent measles outbreak.
The study showed that the first wave of patients during this outbreak in Anuradhapura
comprised infants aged below 12 months. Furthermore, our detailed investigation of
adult patients revealed that the majority showed signs of developing severe complications
such as pneumonia, with a few requiring intensive care.

As part of the measles elimination programme, a MCV was introduced into Sri Lanka’s
EPI in 1984 as a single-dose vaccine administered to infants aged 9 months. Therefore,
individuals aged 29 years or above have not had a routine vaccination for measles.
Most of the individuals in the 11–29 age group only had a single dose of the MCV,
as the guideline for a second dose was only added to the EPI schedule (measles-rubella
vaccine at 3 years) in 2002, following the island-wide measles outbreak of 1999–2000.
In 2012, the EPI schedule was revised again, with the introduction of the measles,
mumps and rubella vaccine to be administered to babies aged 12 months.

The 2013 outbreak of measles in Anuradhapura (and probably in Sri Lanka) could be
due to several reasons. A non-immune cohort of people, born prior to 1984, who have
never been properly vaccinated or had the natural infection were probably the base
population for this outbreak. Another group of susceptible people are those who received
a single dose of MCV between 1984 and 2000 and missed the booster dose in supplementary
programmes. Based on previous data, the proper antibody response could not have happened
in 15 % of the people who received only a single dose of the MCV 14]. Among those who received two doses, a decrease in vaccine effectiveness may have
contributed to this outbreak.

In the initial phase of the outbreak, mainly infants were affected. This could be
attributed to several factors. Recent studies have shown that babies born to mothers
who were immune from measles due to vaccination rather than natural infection are
more susceptible to infection unless they are immunized early. This is due to babies’
relatively low antibody level (or short period of passive immunity) that has transferred
from the mother’s immunity due to being vaccinated 15], 16]. Most importantly, however, is the question of whether the shift of the first dose
of MCV being administered at 9 months of age to 12 months had a significant effect
on the current outbreak. Typically, transferred immunity from mother to baby starts
to wane around the age of 5 to 9 months. If mothers have vaccine-derived immunity,
waning starts much earlier 17]. Recent studies also point to early susceptibility to measles in infants of both
vaccinated women and women with naturally acquired immunity 18]. In Sri Lanka, the mean age of women at childbirth is around 28 years. At the time
of this outbreak, some mothers in this age group may not have ever had measles and
therefore transferred the antibody to their babies; around 50 % might have had vaccine-derived
immunity. Even among vaccinated women, antibody levels could be low 19]. Either way, infants born to these mothers are more susceptible to measles in early
infancy. Even though vaccine effectiveness is 95 % when administered at the age of
12 months 20], the chances of getting measles could be reduced by giving the vaccine at the age
of 9 months, as this is around the age when risk of mortality is higher. Cost benefits
and health outcomes would be much better if the first dose of MCV was administered
at 9 months, even though its effectiveness rate is lower (85 %) 21]. The change in the vaccine schedule means that infants are now susceptible to measles.
It could have been the triggering factor for this outbreak, as it could have increased
the cohort of susceptible population up to the threshold level for the outbreak.

Measles can be associated with complications such as pneumonia, otitis media, encephalitis,
diarrhoea and rare long-term sequel subacute sclerosing pan encephalitis. During the
1999–2000 outbreak, only 2.2 % of patients had features suggestive of pneumonia. Meanwhile,
diarrhoea, otitis media and encephalitis were noted in 16.7, 1.4 and 0.5 % of patients,
respectively. Neurological complications of measles during this outbreak were well
documented 22]. There were five measles-related deaths, however, their actual causes were not documented.
Among the 33 patients admitted to the UMU THA, 30 (90.9 %) and 26 (78.8 %) had pneumonia
and diarrhoea, respectively. Other complications were not observed, but two patients
became tachypnoeic and hypoxic and required ICU care. Most of the measles patients
admitted to the UMU had a normal WCC, thrombocytopenia and elevated liver transaminases,
which can also be noted in dengue fever, Rickettsial infections, atypical pneumonia
and sometimes leptospirosis, all of which are common in Sri Lanka. Therefore, if the
rash goes unnoticed, especially in dark-skinned individuals, measles can be mistakenly
treated as one of the above conditions.