Hand, foot and mouth disease (HFMD) is an important public health problem that has emerged over the past several years. HFMD predominantly infects children under seven years old and occasionally causes severe disease in adults. Among the enteroviruses, enterovirus 71 (EV71) and coxsackievirus 16 (CA16) are the major causative agents of HFMD. In addition, adenovirus cocirculates with enterovirus and has become a possible additional pathogenic factor for HFMD in some cases. Here, we have investigated the neutralizing antibody responses to both enterovirus and adenovirus in adults, with the aim of exploring the prevalence trends of these viruses and the nature of protective immunity in humans to these viral infections. Sera from 391 healthy adults from 21 provinces and cities in China were tested for the presence of antibodies against EV71, CA16, adenovirus human serotype 5 (AdHu5) and chimpanzee adenovirus pan7 (AdC7) using neutralization tests. High seroprevalence rates of EV71, CA16 and AdHu5 were found in the population (85.7%, 58.8% and 74.2%, respectively). The coseropositivity rate of these three viruses was 39.4% (154 of 391), with median neutralizing antibody titers of 80, 40 and 640, respectively, and the neutralizing antibody titer for EV71 was found to be correlated with those of CA16 and AdHu5. AdC7 was found to be a rare adenovirus serotype in the human population, with a seropositivity rate of 11.8%, suggesting that it could be a good choice for a vaccine carrier that could be used in vaccine development.

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The development of a safe and effective vaccine against SARS-CoV-2, the causative agent of pandemic coronavirus disease-2019 (COVID-19), is a global priority. Here, we aim to develop novel SARS-CoV-2 vaccines based on a derivative of less commonly used rare adenovirus serotype AdC68 vector. Three vaccine candidates were constructed expressing either the full-length spike (AdC68-19S) or receptor-binding domain (RBD) with two different signal sequences (AdC68-19RBD and AdC68-19RBDs). Single-dose intramuscular immunization induced robust and sustained binding and neutralizing antibody responses in BALB/c mice up to 40 weeks after immunization, with AdC68-19S being superior to AdC68-19RBD and AdC68-19RBDs. Importantly, immunization with AdC68-19S induced protective immunity against high-dose challenge with live SARS-CoV-2 in a golden Syrian hamster model of SARS-CoV-2 infection. Vaccinated animals demonstrated dramatic decreases in viral RNA copies and infectious virus in the lungs, as well as reduced lung pathology compared to the control animals. Similar protective effects were also found in rhesus macaques. Taken together, these results confirm that AdC68-19S can induce protective immune responses in experimental animals, meriting further development toward a human vaccine against SARS-CoV-2.