Antibody Titres After Vaccination Versus Natural Infection
In a trial, 50 volunteers received two doses of a vaccine (month 0 and month 6); 50 unvaccinated volunteers who had recovered from natural infection were also followed. Antibody titres were: vaccinated group 1:640 at 4 weeks after dose 2, falling to 1:80 at 12 months; naturally infected group 1:1280 at 4 weeks after infection, falling to 1:320 at 12 months. During follow-up, 1 of 50 vaccinated and 8 of 50 naturally infected participants were hospitalised for severe disease. A journalist writes: 'Natural infection gives higher and longer-lasting antibodies, so vaccination is pointless.' The trial was not randomised; the natural-infection group was recruited from a hospital clinic and was on average 12 years older. The numbers are small, and the follow-up period was 12 months.
1. Explain the immunological basis of the higher titre after natural infection and why titre alone does not measure protection.
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Natural infection exposes the immune system to many viral antigens, including ones that may be absent from the vaccine, and the virus replicates for days. This gives more antigen and a longer germinal-centre reaction, producing more plasma cells and higher antibody titres. Antibody titre is only one measure of protection. Protection also depends on the quality of antibodies (affinity and neutralising ability), memory B cells, memory T cells, and mucosal immunity. A higher total titre may include non-neutralising antibodies that do not prevent disease. The groups also differ in age and recruitment, so the titre difference is not a clean comparison of the two immunisation routes.
2. Evaluate the journalist's conclusion using the data, including at least two limitations of the trial.
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The journalist's conclusion does not follow from the data. The naturally infected group had higher titres, but 8 of 50 were hospitalised with severe disease during follow-up, compared with 1 of 50 vaccinated participants. That is a large difference in the outcome that matters most, although the numbers are small. The study was not randomised: the natural-infection group was recruited from a hospital clinic and was on average 12 years older, so age and underlying health could confound the comparison. The follow-up was only 12 months, so 'longer-lasting' is not established. Natural infection also carries the risk of severe illness, death and long-term complications, which vaccination avoids. A better conclusion is that natural infection produced higher antibody titres in this selected group, while vaccination was associated with fewer severe outcomes.
3. Explain why a booster dose at 12 months might raise protection even though the antibody titre is low.
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A low circulating antibody titre at 12 months does not mean the immune system has forgotten the pathogen. Memory B cells and memory T cells persist. On re-exposure to antigen in a booster, memory B cells rapidly differentiate into plasma cells and secrete antibody, and they undergo further affinity maturation, so the secondary response is faster and often produces higher-affinity IgG. T follicular helper cells support this response. A booster can therefore raise protection even when antibody has waned. The timing of the booster should be based on efficacy data and risk groups, not on antibody titre alone.