  Despite achieving strong performance in semi-supervised node classification
task, graph neural networks (GNNs) are vulnerable to adversarial attacks,
similar to other deep learning models. Existing researches focus on developing
either robust GNN models or attack detection methods against adversarial
attacks on graphs. However, little research attention is paid to the potential
and practice of immunization to adversarial attacks on graphs. In this paper,
we propose and formulate the graph adversarial immunization problem, i.e.,
vaccinating an affordable fraction of node pairs, connected or unconnected, to
improve the certifiable robustness of graph against any admissible adversarial
attack. We further propose an effective algorithm, called AdvImmune, which
optimizes with meta-gradient in a discrete way to circumvent the
computationally expensive combinatorial optimization when solving the
adversarial immunization problem. Experiments are conducted on two citation
networks and one social network. Experimental results demonstrate that the
proposed AdvImmune method remarkably improves the ratio of robust nodes by 12%,
42%, 65%, with an affordable immune budget of only 5% edges.
