Being unvaccinated and having a contact history increased the risk of measles infection during an outbreak: a finding from measles outbreak investigation in rural district of Ethiopia

Measles is an acute viral illness caused by a single-stranded RNA virus belongs to the genus Morbillivirus [1]. Measles is one of the most contagious of all infectious diseases with > 90% attack rates among susceptible close contacts [2]. Primarily it is transmitted by respiratory droplets or airborne spray to mucous membranes in the upper respiratory tract or conjunctiva [3]. Measles cases are infectious starting from the prodromal period (when the first symptom appears) to four days after the appearance of the rash.

Measles is characterized by a generalized maculopapular rash, fever, cough, coryza (running nose), conjunctivitis, and photophobia [2]. The incubation period from exposure to the onset of fever is approximately 10–12 days and from the exposure to the onset of rash is 7–18 days [4]. Though many children experience uncomplicated measles, nearly 30% of cases may develop one or more complications that are more common in young children with immune deficiency disorders, malnutrition, vitamin “A” deficiency, and inadequate vaccination [1].

Vaccination has reduced a global measles morbidity and mortality over the last 30 years [5]. Despite these acknowledged sign of progress in morbidity reduction, measles is still not controlled in many parts of the world; particularly in Africa and Asia [6]. During the 2014 and 2015, a total of 296,629 and 206,360 measles cases reported globally [7].

In Ethiopia, the national immunization program was established in 1980 and has been delivering through static and outreach modalities [8]. The current immunization schedule recommends a routine first dose of measles vaccine at the age of nine [9]. As a sero-conversion rate of measles vaccine at 9 months of age is around 85%, a second dose opportunity through supplementary immunization activities (SIAs) or immunization campaigns [9] might be required to protect those children who have never been vaccinated or those who were vaccinated but did not develop immunity [3].

A single dose of national measles vaccination coverage was 76 and 84% in 2013 and 2014, respectively. Ethiopia has adopted the regional goal of measles mortality reduction in 2002 and measles elimination goal in 2012 [9]. Despite several efforts has been made to implement the elimination strategies, the country has been experiencing a number of measles outbreak annually. During the 2014, a total of 16,702 clinically suspected cases were notified to the Ethiopian surveillance system. Of the 13,301 suspected cases reported in 2014, 2373 (18%) were laboratory confirmed, 5692 (43%) were epidemiologically linked, and 5236 (39%) were clinically compatible [9]. Between 2000 and 2014, in the Amhara region, 2412 cases were lab confirmed, 23,842 were epidemiologically linked, and 9699 were clinically compatible [10]. As one of the 11 zones of Amhara, Wag-Himra alone reported 629 measles cases and 24 deaths [11]. As of May 2016, Sekota Zuria district health officials have notified the occurrence of suspected measles outbreak to Wag-Himra Zone public health emergency management (PHEM) department. As a response, a multidisciplinary team was deployed to the area aiming at verifying the existence of an outbreak and to identify its possible causes.

This study intended to assess risk factors associated with measles infection in Zuna kebele of Sekota Zuria district. The measles infection was confirmed after five serum samples were positive for measles specific-IgM antibodies. A measles outbreak was established after three or more lab- confirmed cases were found in the district within a month [3]. Other cases were epidemiologically linked as the outbreak was localized to those laboratory-confirmed cases. None of the cases had a history of traveling to other measles risky areas prior to 7–18 days disease onset but all cases have acquired the infection at least eight days (the minimum incubation period) after the index case has got sick in the kebele they were living. As measles is highly contagious, studies suggested that a single case may infect 17–20 susceptible people and a secondary attack rate near to 75–90% has been reported [4].

No measles related deaths were reported in this study though case fatality from measles is estimated to be 3–5% in developing countries and may reach more than 10% when occurred in nutritionally deprived areas [3]. The absence of death in this study might be due to an effective case management following an early detection of the outbreak and because of the fact that the majority of the cases were adults. The majority of the cases (55.2%) in this study were adults aged ≥15 years. This might be due to the accumulation of unvaccinated adults in the community as they were not targeted for immunization program unlike that of children under-five years. Other findings have also showed a shift in age distribution to adult cases despite the disease was more prevalent among children prior to expansion of immunization programs [1, 12]. The 2014 Ethiopian annual report on measles have showed that 67% of the measles cases of the year were above five years [13]. Another report on epidemiologic data also showed a decreasing proportion of measles cases in children under five and outbreaks continue to occur in most parts of the country with nearly 70% of the reported cases being 15 years and above [9].

The overall incidence rate in the study area was 69.9/10,000 and relatively higher attack rate (93.8/10,000) was observed among 15 years and above. This rate is higher than the attack rate recorded nationally during the measles outbreaks in 2013 and 2014, which was 6.5 and 14.6 per 100,000 population, respectively [13]. Even though the national measles vaccination coverage has generally increased over time (44% in 2003 to 84% in 2014), it is far from the standard coverage in which more than 90% vaccine coverage is required to reduce possible outbreaks [2]. The same is true in the case of Sekota Zuria district in which the vaccination coverage was low (74.2%) and the vaccination rate of the study participants was low (40%).

Our multivariable analysis revealed that previously vaccinated individuals had 83% less risk of acquiring measles infection as compared to unvaccinated individuals. This result is consistent with the theoretically calculated vaccine-preventable fraction which showed 85% protective vaccine efficacy when an individual receives one dose of Measles-Containing Vaccine (MCV) at 9 months of age [3]. This finding is also similar to another study which found that vaccinated individuals had less risk to acquire measles infection [14].

We also found that people who had a contact history with measles cases before 2–3 weeks prior to developing the current infection had a 3.4 times higher risk of acquiring measles infection compared to those people who had no known contact history during the same period. This finding is consistent with the study conducted in Bugna and Kindo Didaya districts of Ethiopia in which susceptible individuals who had a contact history with measles cases had a higher risk of acquiring measles infection as compared to individuals who had no known contacts [15, 16].

As a limitation, we only found 29 measles cases that seems small, however, it has been documented that case-control study is efficient with relatively small sample size in general and especially it is highly recommended in case of an epidemic as compared to other observational studies [17]. This work might also subject to recall bias as some study participants lack immunization cards and were asked to recall their immunization status.

The result of this investigation assures the occurrence of measles outbreak in Sekota Zuria District. Vaccination coverage among cases was lower than among controls. More than half of the total cases were adults’ age ≥ 15 years. High attack rate was observed in older ages. The predominant contributing factors for the occurrence of this outbreak was being unvaccinated and having a close contact history with measles cases. Increasing immunization coverage to ≥90% and strengthening Supplementary Immunization |Activities (SIAs) to refill the routine immunization schedule might reduce the number of susceptible individuals in a population and risk of an outbreak.