Is it reliable to use EventSource to get User's online status?

I have used an EventSource method to get online status of active users on my website. In this following JavaScript code is inserted in every page

var source = new EventSource("set_online.php");

Thus this code is executing set_online.php file continuously.

On server side i.e. in set_online.php following code executed

$query = "UPDATE my_db SET last_active = '{$current_time}' WHERE id = {$_SESSION["id"]}";
                $result = mysqli_query($connection, $query);

Now I have two concerns about this:

1. As Database is updating last_active continuously in realtime, will it affect server load?
2. As connection is open as long as user is on website, will it create vulnerabilities?