Table 2. Increase in titer and symptoms of influenza-like illness in 1,190 travelers from the Netherlands to (sub)tropical countries, October 2006–October 2007.

Strain, subtype, pretravel titer	No. travelers, n = 1,190	Titer >4-fold, no. (%), n = 120*	Influenza-like illness, no. (%), n = 11
A
H3N2
<40	557	32 (6)	6 (19)
>40†	633	12 (2)	1 (8)‡
H1N1
<40	862	24 (3)	1 (4)
>40†	328	5 (2)	0
B/Malaysia
<40	883	20 (2)	2 (10)
>40†	307	3 (1)	0
B/Florida
<40	820	22 (3)	1 (5)
>40†	370	2 (1)	0

*>4-fold rise between pretravel and posttravel titer and posttravel titer >40.
†>40: protective antibodies.
‡Symptoms probably caused by asthma.