Tomcat Access Log does not Show Response Header Value

Question

We are setting a response header in one of our JSP pages, through the following code:

 response.setHeader("X-IA-Session", Integer.toString(clientsession.getS()));

In the access log format pattern (server.xml) we added the snippet:

%{X-IA-Session}o

When checking with WGET the response header is returned correctly, with expected values. And yet, the field above shows up empty in all records.

We are running tomcat 6, on an ubuntu machine.

Any ideas how to resolve this are welcome.