The epidemiology of cases of AGE of viral etiology is a relevant public health issue[20]. Of note, reports on SaV-associated AGE across all age groups have recently increased worldwide[6, 11, 13], which indicated that the cases caused by SaVs may become extensive and global. And Shenzhen is also one of the high-incidence areas of SaV-associated outbreaks recently in mainland China[19, 21]. Human SaVs cause AGE in all age groups in both sporadic cases and outbreaks worldwide as well as predominantly occurs in children[22, 23], which correspond to this study. On the other hand, the outbreak described by the study occured in primary school and expose to vomit was recognized as the cause, which are similar to the previous studies[24, 25].
The investigation showed the difficulty to prove the person-to-person transmission by contact or from aerosols generated by vomit[26–28]. In this study, we investigated the vomiting places and the single factor analysis showed that the participation in the treatment of vomitus and the distance of vomitus were the main factors causing gastroenteritis infection (logistic regression, P < 0.05). It has been proposed that the formation of aerosols that can remain in the air for some time and can then be breathed in and swallowed or through contamination of surface causes the outbreaks[29–31]. The aerosols transmission caused by vomiting could responsible for the majority of gastroenteritis outbreaks which are not caused by foodborne or waterborne outbreaks as described previously[31–34]. Vomit was considered only when exposure was at < 1 m, however, possible exposures at greater distances have also been reported[26, 27]. Our study showed the distance under 1.5 m could cause the infection. Therefore, the study gave the evidence that the greater distance also could cause the cases. Since vomiting is a common symptom in calicivirus outbreaks, vomiting aerosols transmission may exist in most Norovirus outbreaks. Therefore, vomiting aerosols transmission should be valued in the investigation of gastroenteritis outbreaks.
Emerging virus strains often have a risk of causing a pandemic. SaV GII.8 was first identified in two hospitalized children’s samples in Peru, in 2008[35]. In mainland China, GII.8 was first reported in Shenzhen, in 2011[21]. Additionally, in 2019, Xue al. acquired the first GII.8 SaV genome from mainland China[13]. Based on the phylogenetic analysis, the strain SZ08 in this study was classified as a member of GII.8 SaV. In addition, strain SZ08 isolated in 2019 was clustered as independent branch and was different from the strain GZ2014-L231 isolated in 2014 although strain of Shenzhen and strain GZ2014-L231 of Guangzhou were both from the same province, Guagndong. GII.8 strains were also detected in other countries not only in clinical but also in environmental samples[36, 37]. SaVs outbreak caused by GII.8 strains occurred in long term center in previous report[35] and the GII.8 SaV outbreak occurred in primary school. It suggested the GII.8 strain could cause the children and the elderly to infect. Despite the low detection rate, the wide spread distribution of the virus in different countries and wide age groups, make it as important concern to understand its genetic diversity and evolutionary characteristics.
Recombination and amino acid mutations are the two important way of viral evolution not only in SaVs but also in Noroviruses. According to the genomic analysis, there was no recombination occurred in stain SZ08.
Amino acid point mutations in proteins are the important way of viral evolution. In this study, by comparing the differences between the strain SZ08 as new cluster and GII.8 human SaV representative strains of other clusters, more amino acid mutations occurred in VP1 capsid region. GII.8 strain might obtain new antigenicity and receptor binding activity through the variation of amino acid sites in the capsid region, just as it occurs in norovirus, and gained greater susceptibility in the population. This might explain there was the large scale of cases in this SaV outbreak in this study.
In conclusion, vomiting could be one of the transmission mechanisms which may explain a large number of cases of SaV outbreaks. It is noteworthy that SaV-associated diarrhea is generally mild, while large-scale outbreaks by SaV may also occur. In addition, we obtained the genome of a novel GII.8 SaV variant from China and comprehensively analyzed the genomic characteristics. The results of this study could not only provide reference data for SaV researches in the future, but also deepen the understanding of evolution mechanisms of the new GII.8 variant. Constant surveillance is required to monitor the emergence of these strains and will permit the identification of changes in major strains and improve our knowledge of the evolution of SaVs among humans.