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Numerous eruptive variable young stellar objects (YSOs), mostly Class Isystems, were recently detected by the near-infrared Vista Variables in the ViaLactea (VVV) survey. We present an exploratory near-infrared spectroscopicvariability study of 14 eruptive YSOs. The variations were sampled over 1-dayand 1 to 2-year intervals and analysed in combination with VVV light curves. COovertone absorption features are observed on 3 objects with FUor-like spectra:all show deeper absorption when they are brighter. This implies strongeremission from the circumstellar disc with a steeper vertical temperaturegradient when the accretion rate is higher. This confirms the nature of fastVVV FUor-like events, in line with the accepted picture for classical FUors.The absence of Br$\gamma$ emission in a FUor-like object declining topre-outburst brightness suggests that reconstruction of the stellar magneticfield is a slow process. Within the 1-day timescale, 60% of H$_2$-emitting YSOsshow significant but modest variation, and 2/6 sources have large variations inBr$\gamma$. Over year-long timescales, H$_2$ flux variations remain modestdespite up to 1.8 mag variation in $K_s$. This indicates that emission from themolecular outflow usually arises further from the protostar and is unaffectedby relatively large changes in accretion rate on year-long timescales. Twoobjects show signs of on/off magnetospheric accretion traced by Br$\gamma$emission. In addition, a 60% inter-night brightening of the H$_2$ outflow isdetected in one YSO.

Short and long term near-infrared spectroscopic variability of eruptive protostars from VVV