  Many immunization strategies have been proposed to prevent infectious viruses
from spreading through a network. In this study, we propose efficient
immunization strategies to prevent a default contagion that might occur in a
financial network. An essential difference from the previous studies on
immunization strategy is that we take into account the possibility of serious
side effects. Uniform immunization refers to a situation in which banks are
"vaccinated" with a common low-risk asset. The riskiness of immunized banks
will decrease significantly, but the level of systemic risk may increase due to
the de-diversification effect. To overcome this side effect, we propose another
immunization strategy, counteractive immunization, which prevents pairs of
banks from failing simultaneously. We find that counteractive immunization can
efficiently reduce systemic risk without altering the riskiness of individual
banks.
