Event handlers run in blocking manner

I'm running serf through command line on 4 vagrant machines.
The following queries are executed
serf query shell "sleep 10"
serf query shell "ls /usr"

The event handler is defined as
# !/bin/bash

if [ "${SERF_EVENT}" = "query" ]; then
if [ "${SERF_QUERY_NAME}" = "shell" ]; then
while read line; do
$line&
done
fi
fi
I get the following message:

Also, the query response is not seen on remote nodes.