Astronauts who returned from space reactivated dormant viruses, started shedding viral particles in their saliva, and expose a silent risk that could complicate human missions to Mars.

Written by Alisson Ficher

Published on 26/04/2026 at 17:59

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Prolonged exposure to space alters the immune system, favors reactivation of common viruses, and raises alerts for long journeys with potential impact on astronaut health and the operational safety of missions beyond Earth’s orbit.

Astronauts who participated in space missions, both short and long-duration, showed herpesvirus reactivation during and after flight, with viral material detected in saliva and urine samples, according to a review published in the scientific journal Frontiers in Microbiology by NASA-affiliated researchers.

This result indicates that the space environment can compromise the immune response to the point of allowing the reactivation of previously silent viruses in the human body, increasing scientific concern in the face of longer missions planned beyond Earth’s orbit, including journeys to Mars.

Latent viruses reactivate in space

Contrary to what might be supposed, there is no evidence of infection acquired in space, but rather of the reactivation of viruses already present in the body, belonging to an extremely common family that remains latent for years after initial infection, usually without causing noticeable symptoms.

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Frequency increases in longer missions

Analyzing different missions, researchers observed that the occurrence of the phenomenon varies according to the time spent in space, with numbers that help to gauge the biological impact of prolonged exposure to the space environment on astronauts’ immune systems.

In Space Shuttle flights, for example, 47 out of 89 astronauts, equivalent to 53%, shed one or more herpesviruses, while in long-duration missions on the International Space Station, this number rose to 61%, with 14 out of 23 crew members showing the same pattern.

In addition to the higher frequency, a progressive increase in viral load was also recorded during orbital stay, suggesting that the longer the exposure to the space environment, the greater the intensity of the reactivation of these latent viruses tends to be.

The detailed comparison reinforces this trend, showing an increase in varicella-zoster shedding from 41% to 65%, Epstein-Barr from 82% to 96%, and cytomegalovirus from 47% to 61%, when comparing short and prolonged missions.

Asymptomatic cases still concern researchers

Although most cases do not present evident symptoms, researchers emphasize that the phenomenon should not be treated as irrelevant, as there are records of infectious virus recovery in culture and association with clinical manifestations observed during and after missions.

Among these manifestations are atopic dermatitis and skin lesions, as well as episodes compatible with varicella-zoster reactivation, which can evolve into herpes zoster in situations of immune suppression, especially in contexts of greater physiological wear.

Space environment pressures the immune system

The explanation for this behavior involves a complex combination of factors characteristic of the space environment, which act simultaneously on the body and generate a scenario of continuous physiological stress throughout the mission.

Microgravity, cosmic radiation, prolonged confinement, social isolation, circadian rhythm alterations, and sleep deprivation contribute to elevating stress-related hormones, such as cortisol and catecholamines, while simultaneously reducing the effectiveness of the cellular immune response.

This imbalance creates favorable conditions for viruses previously controlled by the body to escape this immune surveillance, resuming replication and becoming detectably circulating in different bodily fluids.

Effects persist after return to Earth

Even after returning to Earth, the effects do not disappear immediately, as in long-duration missions, the continuity of viral shedding was observed for up to 30 days, especially in cases of varicella-zoster and cytomegalovirus.

Comparatively, this period proves to be more prolonged than that recorded in short missions, indicating that the time of exposure to space influences not only the intensity of reactivation, but also the duration of its effects after the end of the mission.

This aspect gains relevance in the planning of future long-duration expeditions, where the absence of rapid evacuation and immediate medical support can transform seemingly controllable clinical events into operational risk factors.

Risks extend to post-mission contact

Another point highlighted by researchers involves the period of readaptation on Earth, when the continuous shedding of infectious viruses can represent a potential risk for newborns, seronegative adults, and immunocompromised individuals, especially in close contact situations.

Thus, the post-mission period ceases to be merely a physical recovery process and begins to require broader monitoring, including immunological evaluation and virological follow-up to prevent possible transmissions or associated complications.

Although astronauts are highly selected professionals and undergo rigorous medical monitoring, the data indicate that the space environment has a significant impact on fundamental biological systems, including those responsible for defense against infections.

When considering long-duration human missions, the need for more robust preventive strategies increases, with protocols capable of monitoring, mitigating, and responding to the reactivation of latent viruses throughout all phases of the space journey.

Astronauts who returned from space reactivated dormant viruses, started shedding viral particles in their saliva, and expose a silent risk that could complicate human missions to Mars.