The efficacy of sodium hypochlorite was assessed against human immunodeficiency virus type 1 suspended in low (8% v/v) or high (80% v/v) concentrations of serum or in a high (80%) concentration of blood. In the presence of 8% serum. 100 p.p.m. available chlorine in the disinfectant test mixture inactivated 3·75 log TCID50 HIV/ml within 30 s. When the test mixture contained 80% serum. 500 p.p.m. available chlorine inactivated more than 4 log TCID50 HIV/ml in 1–2 min. Lower concentrations of available chlorine were unable to inactivate the virus completely. In the presence of 80% blood. 1000 p.p.m. available chlorine in the disinfectant test mixture was unable to inactivate 3·75 log TCID50 HIV/ml. although 2500 p.p.m. available chlorine was able to inactivate at least 1·5 log TCID50 HIV/ml. In all test mixtures, the chlorine rapidly became combined and thus less active. Our results emphasise the importance of cleaning prior to disinfection with sodium hypochlorite since it may prove to be ineffective in the presence of high levels of organic matter. In cases where prior cleaning is impossible, care must be taken to use the higher recommended concentration (a minimum of 10000 p.p.m. available chlorine).

The effectiveness of influenza vaccination in preventing serious illness and death was determined in an elderly population during the influenza epidemic of 1989–90. A retrospective cohort study was carried out using computerized general practitioner records on nearly 10000 patients aged 55 years and over. After adjustment for potential confounding factors, recent immunization was found to have a protective effect of 75% (95% confidence intervals: 21–92%) against death. Protection did not appear to vary with either age or the presence of underlying chronic disease. As the complications of influenza are most common in those with underlying chronic disease, the study findings are consistent with the recommended policy for the use of influenza vaccine in the UK. Further work is necessary to determine the cost-effectiveness of extending immunization to other groups.

The epidemiology of influenza A in Japan was studied during 1979–91 and viruses isolated from reinfections during 1983–91 were analysed, Of 2963 influenza viruses isolated during this period, 922 and 1006 were influenza A(H1N1) and A(H3N2) viruses respectively; the others were influenza B viruses. Influenza A(H1N1) and A(H3N2) caused 5 and 6 epidemics respectively, most accompanied by antigenic drift. Seventeen reinfections with H1N1 and 17 with H3N2 were detected during our study. The primary and reinfection strains isolated from 7 H1N1 and 10 H3N2 cases were studied by haemagglutination-inhibition, and amino acid and nucleotide sequences of the HA1 region of the haemagglutinin. Most of the primary and reinfection strains were antigenically and genetically similar to the epidemic viruses circulating at that time. However, in 4 out of 10 cases of reinfection with influenza H3N2 virus, reinfection strains were genetically different from the epidemic viruses.

Where immunization campaigns locally eliminate measles, it will be important to identify the vaccination policy most likely to prevent future epidemics. The optimum age for vaccination depends on the rate of decline of maternal antibody, because the presence of antibody reduces vaccine efficacy. The first part of this paper contains a quantitative reappraisal of the data on antibody decline and seroconversion rates by age. The decline in maternal antibody protection follows delayed exponentials, with delays of 2–4 months, and subsequent half-lives of 1–2 months. Using this result in an analytical mathematical model we find that the optimal age to administer a single dose of vaccine to children, which is independent of vaccine coverage, lies within the range 11–19 months. We also show that, where the optimal age cannot be met, it is better to err towards late rather than early vaccination. There are therefore two reasons why developing countries, which presently vaccinate during infancy because measles transmission rates are high should eventually switch to the second year of life. The possible gains from two-dose vaccination schedules are explored with respect to both coverage and efficacy. A two-dose schedule will be beneficial, in principle, only when there is a need to increase net vaccine efficacy, after coverage has been maximized with a one-dose schedule.