Rubella virus infection is transmitted by respiratory droplets and causes a generally mild disease characterized by a rash and fever, primarily in children [1]. Although the disease affects both males and females, it is a disease of public health importance in pregnant women causing major problems such as spontaneous abortions, miscarriages, stillbirths, and congenital defects including hearing impairment, heart defects, cataracts known as congenital rubella syndrome (CRS) [2]. From just before conception and during the first 8–10 weeks of gestation, rubella infection may cause multiple fetal defects in up to 90% of cases [3].

Rubella virus, the sole member of the Rubivirus genus in the Togaviridae family, is a positive-polarity ribonucleic acid virus. Both vaccination and natural infection result in life long immunity. This vaccine-preventable disease is among the small number of viral diseases considered to be potentially eradicable [4, 5]. The primary objective of rubella-control programs is prevention of congenital rubella virus infection, which can result in CRS. Cameroon introduced rubella containing vaccine (RCV) into the Expanded Program on Immunization (EPI) in 2015 through a national measles and rubella mass vaccination campaign for children under 15 years [6]

Previous reports about rubella in Cameroon indicated a prevalence of rubella IgG antibodies of 83.9% in women of reproductive age in 1992 [7] and 88.8% in women obtaining antenatal care in 2008 [8]. Recently, a study in the West region reported a prevalence of rubella IgG antibodies of 93.4% among pregnant women [9]. Additionally, a report indicated a prevalence of 1.3% of rubella IgM antibodies in febrile infants in Cameroon [10]. Prior to rubella vaccine introduction by the EPI, rubella IgM seroprevalence was estimated at 9.3% in measles negative sera [11]. There are two reports on CRS in Cameroon, a case report [12] and a study of CRS in school children [13]. Unfortunately, there is no recent report on rubella in pregnant women in the Center region. In addition, there is no report on rubella IgM seroprevalence and IgG avidity in pregnant women in Cameroon. Therefore, the goal of the current study was to re-evaluate rubella infection in pregnant women in Cameroon and to distinguish recent from past rubella infection using rubella IgG avidity testing.

This study investigated the seroprevalence of rubella among Cameroonian pregnant women attending ANC in the Center and South-West regions. Rubella antibodies (IgG and IgM) were detected using ELISA and rubella IgG avidity testing was done to determine the status of rubella infection. The findings showed that 94.4% of the pregnant women were positive to rubella IgG. This confirms previous exposure to wild type rubella virus [1, 16] since, none of the participants in this study had ever received any rubella containing vaccine. The seroprevalence of rubella IgG among pregnant women in this study (94.4%) is higher than that of previous studies in Cameroon where rubella seroprevalence varied from 83.9% to 93.4% among pregnant women attending ANC in hospitals [7, 9]. Other studies in Cameroon show recent rubella virus transmission and reported that persons aged ≥15 years were also more likely to have rubella infection than children under one [11]. Since none of the participants had received any rubella containing vaccine, our results suggest that most of the participants were probably exposed to the virus through natural rubella infection. This high level of rubella seroprevalence in Cameroon likely reflects significant transmission of rubella virus in the country. This is similar to the situation reported in other African countries such as Burkina Faso [17], where a prevalence of 95% was found; Sudan [18] with a prevalence of 95.1%; Nigeria [19] where a prevalence of 91.54% was found and in Tanzania [20], where a prevalence of 90.4% was reported.

Furthermore, as observed in other studies, all the other socio-demographic characteristics were not associated to rubella seroprevalence [16, 17, 21], except for living in the same house with a child with RCV during pregnancy. Even though there is lack of evidence of contact spread from vaccinated individuals [22], living in the same house with a vaccinated infant was protective against rubella infection during pregnancy [16]. However, more information on the timing of rubella vaccination of the children would be needed as the vaccine virus would only be shed for a short time (such as 1 month) after the vaccination was received.

This study shows a lack of immunity against rubella among a small proportion of pregnant women in Cameroon as the proportion of susceptible individuals was 5.6%. Despite the high prevalence of rubella IgG in Cameroon, some pregnant women are still at risk of rubella infection during pregnancy which might lead to the birth of infants with CRS. Presently, some young women of child bearing age are still unprotected from rubella which is consistent with some observations of CRS in Cameroon in the past few years [13]. The main aim of rubella vaccination is to prevent CRS. A measles and rubella containing vaccine was administered for the first time in Cameroon in 2015 to children between 9 months and 14 years during a mass vaccination campaign [6]. We recommend that this strategy employed by the EPI should be re-enforced by also vaccinating young women of child bearing age.

Clinical diagnosis of rubella is unreliable [23], making laboratory diagnosis an important component in the diagnosis of primary rubella infection during pregnancy. In this study, there was no significant association between the presence or absence of clinical signs and rubella IgG positivity. In Ethiopia, pregnant women without maculopapular rash had 2.5 times protective IgG antibody than those who had maculopapular rash [16].

It is recommended to test pregnant women for IgM only if symptoms are evident, if contact with a known case occurred, or if an outbreak is occurring in the region but no pregnant woman should have rubella diagnosed on the basis of a single positive rubella specific IgM test result[24]. Occasionally, rubella IgM may persist for months or years after infection [25]. Additionally, a small percentage of false positive IgM results are seen with all ELISA IgM kits and cross reactivity has been seen with rheumatoid factor and other diseases such as parvovirus B19 [25]. The positive predictive value of IgM tests declines as rubella incidence declines [25] so the need for supplementary assays to accurately diagnose rubella will become more common as Cameroon progresses toward rubella control. This study is the first study to document seroprevalence of rubella IgM in pregnant women in Cameroon. The prevalence of rubella IgM was 5.0% among the pregnant women. This falls within the range of acute rubella infection in Africa observed to vary from 0.3% in pregnant women in Mwanza, Tanzania; to 45.1% in children (1–10 years) in Jos, Nigeria [20].

In cases where a possible false positive rubella IgM result is suspected, one supplemental assay that can be used to distinguish primary infection from remote infection is the measurement of IgG avidity [14]. We recommend that the management of pregnant women presenting with a positive rubella specific IgM result should be followed up. The timing of serum collection is important for pregnant women. If the serum is collected in the first trimester and the avidity is high, rubella infection likely occurred prior to the start of the pregnancy so danger to the fetus is very low. However, if the serum was collected in the second or third trimester, the avidity could already be high if a rubella infection occurred in the first trimester so danger to the fetus cannot be ruled out by high avidity. The interpretation of the results in this study is that past rubella infection had occurred in all IgM positive individuals, as the avidity results were above 50%. Even though all the avidity results were high, it does not completely exclude the possibility of a recent secondary infection. Secondary infections cannot be confirmed without two serum samples with one collected close to onset and a second sample collected one to two weeks later, which was not done in this study. The risk of defects in the fetus due to secondary infection is substantially less than the risk from primary rubella [26]. For IgM positive women presenting with symptoms consistent with rubella, another option for laboratory confirmation of disease is to use ribonucleic acid (RNA) detection by reverse-transcription polymerase chain reaction (PCR) assays on samples such as throat swabs or urine. In this study, 14 urine samples from the IgM positive women were tested with negative results. Real time PCR is most likely to be positive for samples collected within 4 days of disease onset [27]. From this study, no participant presented with an acute rubella infection.

The findings from our study further support the fact that rubella infection represents a public health burden. However, this study was subject to limitations including the small number of IgG negative and IgM positive cases, and no positive result after PCR. Due to limited resources, this cross-sectional survey was done only in two out of ten regions in Cameroon. Despite these limitations, our results highlight the need to investigate rubella during pregnancy, encourage the vaccination of young women of reproductive age presenting with no rubella immunity, and interpret IgM positive results carefully.

The seroprevalence of rubella IgM antibodies was 5.0% in the pregnant participants. No participant presented with an acute/recent rubella infection, as confirmed with high avidity. Before taking any decision to interrupt any pregnancy because of a positive rubella IgM result, IgG avidity testing should be done. Our study reveals that susceptible pregnant women without immunity to rubella are present in the population and they are at risk of bearing children with CRS. Hence, rubella screening of women of child bearing age should be encouraged, those who lack immunity should be vaccinated, and rubella-based surveillance should be strengthened to reduce the burden of rubella and CRS in Cameroon.