Discussion
Our study showed that RSV is an important cause of mild and severe
respiratory infections in children under 5 years of age in CAF2,22,25 and is the first to explore in details the
epidemiology of RSV in the country. Young infants are usually more often
represented in surveillance studies on respiratory infections, due to
the immaturity of their immune system. Compared to the other age groups,
children aged 0-6 months had an increased risk of being hospitalized
regardless of their (RSV) infection status, and the detection rate of
RSV was high during the first months after birth — 13.4% in the 0–6
month age group — and decreased with age, to 4.0% in children 4-5
years old. Previous reports have shown higher infection rates during the
first year of life linked to primary infections7,12,26-28. Within the first year of life, a
significant difference in hospitalization risk according to the month of
birth was shown 29, linked to infant age and levels of
maternal antibodies elicited by more or less recent RSV exposure of the
mother 30,31. RSV infection leads to humoral immune
response also in young infants but antibody titers are likely not high
enough to confer full protection. This allows reinfections that will
then boost humoral immune response 32,33 and
explaining why RSV is still detected in older age groups, albeit with
lower incidence (4.0-7.5% in CAF). Our analyses also showed that male
patients developed more frequently more severe symptoms leading to
hospitalisation (33.3% vs 29.6% in female) and were more susceptible
to RSV infection (9.5% vs 6.4%), as already reported previously31,34. Hormonal influence on the immune system35 and/or airway anatomical differences31 have been proposed to contribute to sex differences
in susceptibility to respiratory infections.
Our study showed an overall prevalence of RSV of 8.0% in the period of
2015-2018, with significant annual (6.4%-10.6%) and seasonal (12.7%
in rainy season vs 3.0% in dry season) fluctuations. While RSV
circulation is high during the winter in temperate climates12, a high RSV incidence usually coincides with the
rainy season in African countries with a tropical climate, such as in
Kenya 36, Cameroon 37, Senegal25 and Ghana 38. Also in CAF most
cases were reported in November in a study from 201020. RSV seasonality likely depends on climatic factors
such as relative humidity, temperature and UV radiation that influence
the infectivity of viral particles and stability of aerosols39 or host factors such as overall increased
susceptibility to infectious diseases in winter due to lower levels of
vitamin D 40-42. Deviations from overall seasonality
patterns, as observed in 2017 in CAF where RSV incidence was
significantly higher with a peak 3-5 months earlier than usual, have
been documented previously. In Germany, an earlier start of the RSV
season as compared to the average tends to be linked to higher disease
incidence 12. In Switzerland and Finland, seasons with
lower incidence and later start alternate with a 2 year-cycle with
outbreaks that start earlier in the season 43,44, have
a higher incidence and increased hospitalisation rates43. Outbreak periodicity is likely influenced by the
level of herd immunity developed during previous seasons and/or RSV
genetic diversity. In China, RSV-A dominated seasons started earlier and
lasted longer than RSV-B dominated seasons 45.
Although RSV incidence in 2016 in CAF did not differ from 2015 (p=0.931)
or 2018 (p=0.345), RSV-B was the predominant subtype in that year,
likely affecting herd immunity against RSV-A and thus allowing earlier
and wider RSV-A circulation in 2017. Long-term RSV surveillance in CAF
is needed to better understand the periodicity of RSV-A and B dominance
that differs between countries 45-47, and to refine
data on RSV season onset, peak and duration in the country.
Phylogenetic analyses, based on partial glycoprotein G sequences,
revealed an RSV-A genotype replacement in CAF. While NA1 was the
dominant genotype in 2015, its detection rates decreased and it was not
found any longer in 2018. RSV temporal genotype replacement is well
documented 48-50 and is facilitated by viral evolution
and immune selection 51. Notably, RSV-A ON1 and RSV-B
BA genotypes, with a 72 or 60 nt duplication located in the second
hypervariable region of the G protein 4,19,50, have
spread worldwide. They have become predominant across different
continents 2,4,19,38,50,52, likely due to a fitness
advantage conferred by the duplication 53. In
addition, temporal strain replacement within a genotype also occurred in
CAF. Within ON1, one large cluster of 79 identical strains in 2017-2018
suggested sustained local transmission. Smaller clusters of identical
strains identified during two consecutive seasons (2016-2017 or
2017-2018) or detected in 2016 and 2018 also suggested that these
strains were maintained in the country over time. Sporadic cases
detected during the dry season, outside the main RSV seasons, may
maintain RSV transmission in the population between two outbreaks,
without the need for virus re-introduction 54.
However, CAF strains interspersed with RSV strains from abroad,
suggesting that new RSV strains are also introduced in the country.
Maintaining surveillance in CAF as well as in neighbouring countries
while increasing the sequencing effort, both concerning the number of
strains and the sequence length, will help to characterize the
importance of local versus imported strain circulation in the country.
While the NA1 genotype detected in CAF showed limited polymorphism14,55 potentially contributing to its elimination in
CAF 12, the ON1 and BA9 CAF strains showed a higher
degree of polymorphism. Among the 33 substitutions observed in the ON1
CAF strains compared to the prototype strain, L274P substitution has
been linked to the RSV evasion from antibodies 2.
Among the 23 mutations in BA9 strains, the genotype-specific
substitutions L223P, S247P, I281T and H287Y 56,57 were
also identified in CAF strains. Gain (D245N substitution in 3 unique NA1
and D273N in 3 unique BA9 strains) or loss (positions 318-320 in 4
unique ON1 strains, 296-298 or 310-312 in 3 BA9 strains) of N-linked
glycosylation might potentially affect antigenicity58,59.
Clinical manifestations of RSV infections vary and can include symptoms
of both upper and lower respiratory infections 60. In
our study, RSV was significantly associated with dyspnea, wheezing,
chest indrawing and inability to feed as reported before26, while its association with fever or cough could
not be assessed due to the use of WHO case definitions developed for
influenza surveillance, constituting a limitation of our study25. Indeed, a substantial number of RSV infected
patients, especially young infants 26,61, do not
develop fever, while this symptom is part of the ILI and SARI case
definitions. Using ARI and extended SARI definitions should increase
sensitivity for RSV case identification 26,61.
Moreover, screening efficiency can be greatly improved by using
real-time RT-PCR. While similar detection rates were found in the USA
(6.1%, 62) or in Kenya (8%; 36)
when using conventional RT-PCR, much higher detection rates were
reported in Germany (23%; 12) and in Ghana (23%;38), when using more sensitive real-time RT-PCR assays63. Therefore, combining revised RSV surveillance
criteria and a more sensitive real-time RT-PCR screening approach will
improve the sensitivity of RSV detection in the country in the future.