153 2013;28(2) South Afr J Epidemiol Infect Original Research: A sero-survey to identify the window of vulnerability to measles in infants As infants lose maternal measles antibodies (MMAs), they experience periods when their antibody levels are insuficient to protect them against measles. A prospective study was carried out at the University of Maiduguri Teaching Hospital. Sera collected from neonates at birth, and at six weeks, three months, six months and nine months of age, were analysed for MMAs by enzyme-linked immunosorbent assay. Seventy-seven neonates were enrolled. Of these, 73 (94.8%) had protective MMAs at birth. This igure declined to 36 (46.8%), 28 (36.4%), 13 (16.9%) and 4 (5.2%) at six weeks, three months, six months and nine months of age (χ 2 = 154.264, p-value = 0.000). Protective MMAs at birth waned rapidly, resulting in an early window of vulnerability to measles by the age of six months. Protecting infants with early measles immunisation with potent, safe vaccines are recommended. Peer reviewed. (Submitted: 2012-05-11. Accepted: 2012-09-14.) © SAJEI South Afr J Epidemiol Infect 2013;28(3):153-155 A sero-survey to identify the window of vulnerability to measles in infants in north-eastern Nigeria BU Ahmadu, GM Ashir, MG Mustapha, Y Mava, IH Saidu, JP Ambe Baba Usman Ahmadu, MBBS, MHPM (Health Planning and Management), FMCPaed (Paediatrics), Consultant Paediatrician, Department of Paediatrics, Federal Medical Centre, Yola, Adamawa State, Nigeria. Visiting Paediatrician at Abubakar Tafawa Balewa University Teaching Hospital, Bauchi, Nigeria Garba Ashir, MBBS, FWACP (Paediatrics), Consultant Paediatrician and Senior Lecturer, Department of Paediatrics, University of Maiduguri Teaching Hospital, Maiduguri (UMTH), Nigeria Modu Mustapha, MBBS, FWACP (Paediatrics), Consultant Paediatrician and Senior Lecturer, Head of Department, Paediatrics, (UMTH), Nigeria Yakubu Mava, MBBS, FWACP (Paediatrics), Consultant Paediatrician and Senior Lecturer, Head of Department, Paediatrics, Bingham University Jos, Nigeria Isa Saidu, MBBS, Registrar (Paediatrics), Department of Paediatrics, UMTH, Nigeria Jose Ambe, MBBS, FMCPaed (Paediatrics),Consultant Paediatrician and Professor, Department of Paediatrics, UMTH, Nigeria E-mail: ahmadu4u2003@yahoo.com Keywords: maternal measles antibodies, window of vulnerability, neonates Introduction Since the introduction of an inexpensive and highly effective measles vaccine in the 1960s and the global prioritisation of measles control initiatives, the burden of the disease has signiicantly reduced worldwide. 1 In 1997, there were 36 million cases, and more than one million deaths occurred worldwide, but measles now accounts for an estimated 164 000 deaths per year globally. 2 Yet, it still persists as a public health threat. In Africa, the number of cases increased from 36 000 in 2009 to 172 824 in 2010, and outbreaks were reported in countries with successful measles control programmes. 3 During a two-year study, Ashir et al reported 11.2% cases of measles in Maiduguri, Nigeria in 2009. 4 A notable proportion of measles occurs in infants before the age of nine months, which is the age at which the measles vaccination is currently recommended in most developing countries. 5 The occurrence of measles in these infants results from numerous factors, including the high transmissibility of the measles virus and increasing rates of vaccine refusal because of ignorance and socio-cultural beliefs, especially in developing countries, which deprive children of access to immunisation services. 4,6 The current measles vaccine does not elicit high antibody levels when administered to infants who are younger than nine months of age. 7 Maternal measles antibodies (MMAs) that are still present in infants could neutralise the measles vaccine, thereby making it ineffective before one year of age. Finally, while MMAs, at adequate levels, can protect infants early in life, multiple factors inluence the quantity of measles antibodies that cross the placenta to the foetus. 8 As a result, for several months many infants are vulnerable when their MMAs fall below protective levels against the measles virus. At the same time, these low MMAs may interfere with antibody production in response to the measles vaccine. During this window of vulnerability, young infants may develop severe or fatal measles, if exposed to the measles virus. 9 In the 1990s, vaccination of six-month-old infants with a high- dose measles vaccine was explored as a strategy to overcome MMAs and enable successful measles immunisation of infants during this window period. 10 However, excess mortality in female infants, which was believed to be as a result of their low antibody-dependent cellular cytotoxicity response, led to abandonment of that approach. 11,12 The decision to forsake the use of a high-dose measles vaccine may have been too hasty because excess female mortality in recipients of the high-dose measles vaccine was not a consistent inding. For instance, excess female mortality was not reported in Gambian subjects who received the high-dose measles vaccine. 12 A possible explanation for this inding could be the presence of MMAs