The minimum protective level of antibodies in smallpox

Area of Science:

• Virology
• Immunology
• Epidemiology

Background:

• Smallpox, a eradicated viral disease, remains a public health concern.
• Understanding immune responses, particularly antibody levels, is crucial for assessing protection.
• Previous studies highlight the importance of vaccination in preventing smallpox.

Purpose of the Study:

• To investigate the relationship between antibody levels and smallpox infection in contacts of cases.
• To evaluate the protective effect of vaccination against smallpox disease.

Main Methods:

• Blood samples were collected from 57 contacts of 6 confirmed smallpox cases.
• Samples were tested for haemagglutination-inhibiting (HI) and neutralizing antibodies.
• Infection status and vaccination history were recorded for all contacts.

Main Results:

• All 6 unvaccinated contacts who developed smallpox had low neutralizing antibody titres (≤10).
• 6 unvaccinated contacts with similar low antibody levels did not contract smallpox.
• None of the 41 vaccinated contacts developed smallpox, irrespective of their antibody levels.

Conclusions:

• Smallpox vaccination appears to provide robust protection against disease, even with low antibody titres.
• Low neutralizing antibody levels in unvaccinated individuals may indicate increased susceptibility but not certainty of infection.
• Further research into correlates of protection is warranted.