**4.2 Live attenuated vaccines**

#### *4.2.1 BCG*

The BCG-Denmark strain was tested in randomized-controlled trials (RCT) in infants who normally did not receive the BCG vaccine at birth. These studies carried out in Guinea-Bissau demonstrated that vaccination at birth was associated with lower neonatal mortality, especially due to neonatal sepsis, respiratory infections, and fever [111, 115]. In these lines, a meta-analysis commissioned by the WHO concluded that BCG administration during the first month of life reduces all-cause mortality by 30% [116]. In these studies authors did not discriminate the etiology of infection (bacterial vs. virus); therefore, a reduction in viral infections may explain, to some extent, this result. However, in two studied carried out in India in neonates with BCG-Russian strain no such effect was observed [117]; suggesting that different immunological effect of diverse BCG strains may account for these discrepancies. A study carried out in infants to assess the impact of BCG vaccination on the incidence of RSV infection suggested a possible protective role for BCG vaccination against acute lower respiratory tract infection [118]. Other clinical studies have provided evidence suggesting a protective role for BCG on secondary viral infections [79]. In this regard, the impact of BCG vaccination on viral infection in human healthy volunteers has been assessed using the live attenuated yellow fever vaccine (YFV) as a model of viral human infection [76]. BCG vaccination induced epigenetic reprogramming in human monocytes, and these modifications correlated with IL-1β upregulation and the reduction of viremia, all these features being the hallmarks of trained immunity [76].

Similar protective effect of BCG was observed in several studies in elderly people regarding respiratory tract infections. BCG vaccination in subjects of 60–75 years old once a month for three consecutive months resulted in reduction of acute upper respiratory tract infection, concomitant to significant increase in IFN-γ and IL-10 compared with those receiving placebo [119]. A recent randomized trial of BCG vaccination was carried out in elderly patients (age ≥ 65 years) returning home from hospital admission, these subjects are at high risk to develop infections. The BCG vaccination increased the time to first infection (primary outcome) and decreased the incidence of a new infection [120]. Besides, results demonstrated that BCG vaccination resulted in lower number of infections of all causes, especially respiratory tract infections of probable viral origin, although no discrimination was made between respiratory tract infections caused by bacteria or viruses.

BCG has also been shown to enhance the response to vaccines directed against viral infections [79]. A clinical study in healthy volunteers demonstrated that BCG administration prior to influenza vaccination increases antibody titers against the 2009 pandemic influenza A (H1N1) vaccine strain, concomitantly with an enhanced IFN-γ production to influenza antigens compared with the control group [121].

### *4.2.2 Influenza vaccine*

The cold-adapted, live attenuated influenza vaccine (CAIV) has been shown to provide non-specific cross-protection against RSV in an experimental model of infection [122].

In a randomized pilot study conducted in healthy volunteers receiving a trivalent influenza vaccine, cytokine responses against unrelated pathogens were observed [121]. During the 2003–2004 influenza A (H3N2) outbreak, an open-labeled, nonrandomized vaccine trial was carried out in children 5 to 18 years old. Subjects received either trivalent live attenuated or inactivated influenza vaccine. Live attenuated influenza vaccine but not trivalent inactivated vaccine was effective in children administered during influenza outbreak, despite the dominant circulating influenza virus was antigenically different from the vaccine strain [123].

*Trained Immunity-Based Vaccines: A Ready-to-Act Strategy to Tackle Viral Outbreaks DOI: http://dx.doi.org/10.5772/intechopen.95765*

### *4.2.3 Measles vaccine*

Measles vaccine (MV) is among the live vaccines that have been shown to have beneficial effects reducing all-cause mortality [124]. Randomized clinical trials and observational studies from low-income countries have concluded that measles vaccination is associated with decreased overall mortality and morbidity [100]. However, a systematic review carried out by Higgins and colleagues has pointed out that most of these studies were considered at high risk of bias [116]. Nevertheless, MV seems to induce a transient suppressive effect on both the lymphoproliferative and innate response evaluated in peripheral blood mononuclear cells (PBMCs) from children, with slight increase in innate immune response, measured by non-specific cytokine production [100]. It has been reported that following measles vaccination, the *ex vivo* production of both innate (IL-6 and TNF-α) and adaptive (IFN-γ and IL-2) cytokines decreases for 2 weeks, but levels of IL-2, IL-6 and IFN-γ are increased at day 30 post vaccination compared with baseline [125]. Differences in males and females have been reported, where girls seem to receive stronger beneficial effects. In this regard, a study of MV-specific innate responses following MMR vaccination found higher TNFα, IL-6 and IFN-α secretion, cytokines associated to trained immunity, in adolescent girls than boys [126].
