Table 3. Fluctuating VCA IgA titres of non-NPC subjects during follow-up.

	Negative seroconversion				Positive seroconversion
	Seropositive group		Seronegative group		Seropositive group		Seronegative group
Years	n	%	n	%	n	%	n	%
1987–88	1705	65	0		0		1153	12
1988–90	489	70	44	89	1025	15	376	5
1990–92	374	45	23	65	1946	23	557	8
1992–93	393	47	55	67	876	18	728	5
1993–94	382	24	64	44	1000	24	750	7
1994–95	387	53	70	67	440	17	546	5
1995–98	133	68	27	59	401	14	344	6
1998–99	74	66	30	80	362	7	312	4
Mean±s.d.	3937	57±17	313	66±14	6050	19±4	4766	7±3

NPC=nasopharyngeal carcinoma; s.d.=standard deviation; VCA IgA=viral capsid antigen IgA.

Pairs of consecutive serum samples (n) were taken at indicated intervals during follow-up from non-NPC subjects, who were initially screened at the time of enrollment as having ⩾cutoff VCA IgA titre (seropositive group) or < cutoff VCA IgA titre (seronegative group). Fluctuating antibody level was indicated, when antibody levels rose to or beyond cutoff titre (positive seroconversion) or declined below cutoff titre (negative seroconversion). Pairs of consecutive serum samples (n) were taken at indicated intervals during follow-up from non-NPC subjects, who were initially screened at the time of enrollment as having ⩾cutoff VCA IgA titre (seropositive group) or < cutoff VCA IgA titre (seronegative group). Mean percentage of negative seroconversion of either seropositive or seronegative group is significantly higher than the corresponding values of mean percentage of positive seroconversion (χ²⁼1536.33, P<0.001; χ²=1026.66, P<0.001); mean percentage of positive seroconversion of the positive group is significantly higher than that of the seronegative group (χ²⁼299.33, P<0.001); and mean percentage of negative seroconversion is significantly higher for the seronegative group than seropositive group (χ²=13.82, P<0.01).