Explained: Assessing Covid-19 reinfection risks for the young

The authors say that despite previous infection and the presence of antibodies, vaccination is still necessary to boost immune responses, prevent reinfection, reduce transmission, and that young people should take up the vaccine wherever possible.

The authors say that despite previous infection and the presence of antibodies, vaccination is still necessary to boost immune responses, prevent reinfection, reduce transmission, and that young people should take up the vaccine wherever possible.

A past Covid-19 infection does not completely protect against reinfection in young people, according to a new observational study published in The Lancet Respiratory Medicine.

The study was among more than 3,000 healthy members of the US Marines Corps, most of whom were in the age group 18-20. Of 2,346 Marines followed long enough for this analysis of reinfection rate, 189 were seropositive and 2,247 were seronegative at the start of the study, between May and November 2020. Around 10% of participants who were previously infected with SARS-CoV-2 (19 out of 189) became reinfected, compared with new infections in 50% (1,079 out of 2,247) of participants who had not previously been infected.

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Implications beyond cohort

Although the study was in young, fit, mostly male Marine recruits, the authors believe that the risk of reinfection found in their study will apply to many young people, but that the exact rates of reinfections will not be applicable elsewhere. This is owing to the crowded living conditions on a military base and close personal contact required for basic training likely contributing to a higher overall infection rate than seen elsewhere.

For example, a study of 4 million people in Denmark also found that the risk of infection was five times higher in people who had not before had Covid-19. Only 0.65% of people who had Covid-19 during Denmark’s first wave tested positive again during the second wave, compared with 3.3% of people who tested positive after initially being negative. In addition, a preprint study including British healthcare workers found that those who had been not previously infected had a five times higher risk of being infected than people who had a past infection.

Antibodies & viral load

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To understand why reinfections occurred in the study of Marine recruits, the authors studied the reinfected and not infected participants’ antibody responses. They found that, among the seropositive group, participants who became reinfected had lower antibody levels against the SARS-CoV-2 virus than those who did not become reinfected. In addition, in the seropositive group, neutralising antibodies were less common.

Comparing new infections between seropositive and seronegative participants, the authors found that viral load in reinfected seropositive recruits was on average only 10 times lower than in infected seronegative participants, which could mean that some reinfected individuals could still have a capacity to transmit infection, but the authors note that this will need further investigation.

Anuradha Mascarenhas

Anuradha Mascarenhas is a Senior Editor at The Indian Express, based in Pune. With a career spanning three decades, she is one of the most respected voices in Indian journalism regarding healthcare, science and environment and research developments. She also takes a keen interest in covering women's issues . Professional Background Education: A gold medalist in Communication and Journalism from Savitribai Phule Pune University and a Master’s degree in Literature. Author: She authored the biography At The Wheel Of Research, which chronicles the life and work of Dr. Soumya Swaminathan, the former Chief Scientist at the WHO. Key Focus: She combines scientific accuracy with storytelling, translating complex medical research into compelling public and human-interest narratives. Awards and Recognition Anuradha has won several awards including the Press Council of India's national award for excellence in journalism under the gender based reporting category in 2019 and the Laadli Media award (gender sensitivity -2024). A recipient of the Lokmat journalism award (gender category-2022), she was also shortlisted for the RedInk awards for excellence in journalism-2021. Her debut book At The Wheel Of Research, an exclusive biography of Dr Soumya Swaminathan the inaugural chief scientist of World Health Organisation was also nominated in the Popular Choice Category of JK Paper AUTHER awards. She has also secured competitive fellowships including the Laadli Media Fellowship (2022), the Survivors Against TB – New Research in TB Media Fellowship (2023) and is part of the prestigious 2025 India Cohort of the WomenLift Health Leadership Journey.” Recent Notable Articles (Late 2025) 1. Cancer & Specialized Medical Care "Tata Memorial finds way to kill drug-resistant cancer cells" (Nov 26, 2025): Reporting on a breakthrough for triple-negative breast cancer, one of the most aggressive forms of the disease. Discipline, diet and purpose; How a 97-year-old professor defies ageing'' (Nov 15, 2025) Report about Prof Gururaj Mutalik, the first Head of Department at Pune's B J Government Medical College who at 97 credits his longevity to healthy habits and a strong sense of purpose. 2. Environmental Health (The "Breathless Pune" Series) Long-term exposure even to 'moderate' air leads to chronic heart, lung, kidney issues" (Nov 26, 2025): Part of an investigative series highlighting that even "safe" pollution levels are damaging to vital organs. "For every 10 µg/m3 increase in PM2.5 level, there was 6-8% jump in medicine sales" (Nov 23, 2025): Using commercial data to prove the direct link between air quality and respiratory illnesses in Pune. 3. Lifestyle & Wellness News "They didn't let cancer, diabetes and heart disease stop them from travelling" (Dec 22, 2025): A collaborative piece featuring survivors who share practical tips for traveling with chronic conditions. At 17, his BP shot up to 200/120 mmHG; Lancet study flags why child and teen hypertension doubled between 2000 and 2020'' (Nov 12,2025)--A report that focusses on 17-year-old-boy's hypertensive crisis and reflects the rising global trend of high blood pressure among children and adolescents. 4. Scientific Recognition & Infrastructure For promoting sci-comm, gender diversity: IUCAA woman prof highlighted in Nature" (Nov 25, 2025): Covering the global recognition of Indian women scientists in gender studies and physics. Pune researchers find a spiral galaxy like the Milky Way from early universe'' (December 3, 2025)- A report on how Indian researchers discovered a massive galaxy that existed when the universe was just 1.5 billion years old , one of the earliest to have been observed so far. Signature Beat: Health, Science & Women in Leadership Anuradha is known for her COVID-19 reportage, where she was one of the first journalists to provide detailed insights into the Covishield and Covaxin trials. She has a dedicated interest in gender diversity in health and science, often profiling women researchers who are breaking the "leaky pipeline" in STEM fields. Her writing style is scrupulous, often featuring interviews with top-tier scientists and health experts from various institutions. ... Read More