WASHINGTON (BLOOMBERG) - Young adults previously infected with Covid-19 are still at risk of catching the disease again, according to a study of US military personnel that highlights the importance of vaccinating even those who have tested positive for the virus.
About 10 per cent of 189 people who had been infected once, most of them 18- to 20-year males, later became reinfected, according to a study of 2,346 US Marine Corps members published on Thursday (April 15) in The Lancet Respiratory Medicine journal. The study took place between May and November 2020.
Access to Covid-19 vaccines has become less secure in Europe as many countries have restricted the use of the AstraZeneca vaccine to their older populations due to its link with the rare form of blood clotting. Some, including Denmark have halted the use of the shot, while in the US, public health advisers have paused the use of Johnson & Johnson's vaccine on similar grounds.
"One of the main takeaways from this study is the need for ongoing vaccination even if you've been previously infected through natural infection," said Andrew Letizia, deputy director for the Naval Medical Research Centre's Infectious Diseases Directorate and lead researcher on the study.
While antibodies that arise as a result of infection are protective, he said, the immunity is still somewhat fallible. The findings were all the more urgent, as around two-thirds of the participants who became infected were asymptomatic, the researcher said.
"An otherwise healthy young individual who doesn't even realise they're sick can clearly unknowingly spread the infection to another person that's potentially far more susceptible to a poorer outcome," he said.
The daily activities of a Marine, living in close quarters with their peers, may offer more opportunities for re-infection than in otherwise normal life. But a number of other reports, including a preprint study of US health-care workers, confirm that some level of re-infection can take place.
Letizia said he hoped that the study would be able to help those modelling the spread of the virus to ascertain the proportion of the population that would need to be vaccinated to prevent more circulation.