It's important to understand what the terms mean in order to answer the question. An IDS(Intrusion detection system) detects malicious network traffic but does not actively prevent it, while an IPS(intrusion prevention system) reacts to block the traffic.
Now even if an IPS was in place, an antivirus would act as another layer of protection even if the IPS failed to stop the malicious traffic. In addition an IPS would be no use against attacks which did not come in through the network gateway i.e. from within the local network or via removable storage devices.
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