Abstract
Soon after its identification, norovirus (NoV) has been indicated as one of the most common causes of outbreaks of acute gastroenteritis (AGE) and sporadic acute diarrhea episodes in subjects of any age. In 2016 the World Health Organization stated that the development of a NoV vaccine should be considered an absolute priority. Unfortunately, the development of an effective NoV vaccine has proven extremely difficult, and only in recent years, some preparations have been tested in humans in advanced clinical trials. In this paper, reasons that justify efforts to develop a NoV vaccine, difficulties encountered during NoV vaccine development, and NoV vaccine candidates will be discussed. In recent years, identification of some NoV antigens that alone or in combination with other viral antigens can induce a potentially protective immune response has led to the development of a large series of preparations that seem capable of coping with the problems related to NoV infection. Epidemiological and immunological studies have shown that multivalent vaccines, including both GI and GII NoV, are the only solution to induce sufficiently broad protection. However, even if the road to formulation of an effective and safe NoV vaccine seems to be definitively traced, many problems still need to be solved before the total burden of NoV infections can be adequately controlled. Whether currently available vaccines are able to protect against all the heterologous NoV strains and the variants of the most common serotypes that frequently emerge and cause outbreaks must be defined. Moreover, as performed clinical trials have mainly enrolled adults, it is mandatory to know whether vaccines are effective in all age groups, including younger children. Finally, we must know the immune response of immunocompromised patients and the duration of protection induced by NoV vaccines. Only when all these problems have been solved will it be possible to establish an effective immunization schedule against NoV infection and calculate whether systematic vaccination can be cost effective.
Highlights
Soon after its identification in 1972, norovirus (NoV) has been indicated as one of the most common causes of outbreaks of acute gastroenteritis (AGE) and sporadic acute diarrhea episodes in subjects of any age [1]
Considering that intramuscular administration could induce a more rapid and higher antibody response than theintranasal vaccine, a parenteral vaccine was prepared. It was developed by Takeda Pharmaceutical Company Limited and was based on a Norwalk Genogroups I (GI).1 strain virus-like particles (VLPs) that cross-reacts with other GI.1 strains and a consensus of 3 GII strains [2006a (Yerseke), 2006b (Den Haag), and 2002 (Houston)] to increase the protection against GII strains as much as possible [50]
Identification of some NoV antigens that alone or in combination with other viral antigens can induce a potentially protective immune response has led to the development of a large series of preparations that seem capable of coping with the problems related to NoV infection
Summary
Soon after its identification in 1972, norovirus (NoV) has been indicated as one of the most common causes of outbreaks of acute gastroenteritis (AGE) and sporadic acute diarrhea episodes in subjects of any age [1]. The incidence of disease is higher in younger children and in adults ≥65 years old This fact highlights that among healthy subjects, those who have physiologically lower defenses are those with the highest risk of NoV AGE and those for whom effective preventive measures are urgently needed. Symptomatic infection due to GII genotypes was observed only as primary infections or when a previous episode was due to a different GII genotype It is not precisely defined how long the effective protection lasts; if there are differences between adults and children, the youngest; and if antigenic drift plays a role, as in the case of influenza virus, to reduce protection evoked by previous infections with the same genotype. Challenge studies are carried out with viral inocula significantly higher than those needed to cause natural infection, and this difference can modify the immune response, leading to incorrect conclusions
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