What happens to your immune system in space

What changes and why

The immune system does not simply switch off in space; it becomes unbalanced. Some defenses weaken while others become overactive [1].

• Some defenses weaken. In 23 astronauts on 6-month station missions, the ability of T cells (white blood cells that direct the immune response and kill virus-infected cells) to respond was reduced for the whole mission, and production of several immune signaling molecules (cytokines) stayed lower [2].
• Other defenses become overactive. Parts of the body’s first-line (innate) immunity seem heightened, and some crewmembers have persistent allergy-like (hypersensitivity) reactions during flight [1].
• Stress plays a part. Stress hormones such as cortisol and adrenaline rise during spaceflight; together with weaker cell-mediated immunity, this is thought to help sleeping viruses wake up [3].

Sleeping viruses wake up

Most adults carry herpes-family viruses that stay dormant (latent) in the body for life, including the chickenpox and shingles virus (varicella-zoster virus), Epstein-Barr virus and cytomegalovirus. They can reactivate, especially in flight, and are shed in saliva or urine [3].

• In 23 long-duration astronauts, 22 shed one or more of these viruses at some point before, during or after flight, compared with 2 of 20 people on the ground. Shedding was more common and heavier in flight [4][5].
• Even on 12- to 16-day Shuttle flights, 14 of 17 astronauts shed Epstein-Barr virus; 7 also shed the chickenpox virus and 8 cytomegalovirus [6].
• Shedding is usually symptom-free. In a few cases live virus was found together with skin rashes or eczema-like symptoms during and after flight [3].

Rashes and allergies

In medical records of 46 crewmembers on roughly 6-month missions, skin rashes were the most common immune-related problem, followed by cold-like upper respiratory symptoms. Almost half (46%) had at least one “notable” event that lasted a long time or did not respond to treatment, and 40% of these were rashes or hypersensitivity reactions [7]. In one case, a rash and allergy-like symptoms began on day 17 of a 191-day mission, lasted the whole mission and resisted several treatments [4].

It is still unclear whether these immune changes actually make astronauts more likely to get sick [4].

Timeline: days to a Mars mission

1. First daysShort flights

   Changes start fast. After a 3-day private orbital flight [8], 18 immune signaling molecules (cytokines and chemokines) had changed, and the crew’s blood cells showed a gene-activity “spaceflight signature” that was also found in the year-long Twins Study run by the U.S. space agency, the National Aeronautics and Space Administration (NASA) [9]. Virus shedding was common on 12- to 16-day Shuttle flights [6].

2. WeeksFirst months

   Problems can surface early: in one crewmember a stubborn rash with allergy-like symptoms began on day 17, at the time of the first major mission stressor [4].

3. About 6 monthsTypical station stay

   Reduced T-cell function persisted for the whole of 6-month missions [2]. Virus shedding did not settle down on station missions; it became more frequent: chickenpox virus from 41% (Shuttle) to 65% (station), Epstein-Barr virus from 82% to 96% and cytomegalovirus from 47% to 61% [5][3].

4. Mars-length missionsAbout 2 to 3.4 years (730–1,224 days)

   Researchers warn that longer missions and deep-space radiation may add to these changes and raise clinical risk [1]. NASA rates the in-mission risk for Mars as one that requires mitigation, and the long-term risk as one still needing better characterization [10].

5. Back on EarthLanding and after

   After station missions, chickenpox virus and cytomegalovirus were still found 30 days after landing, the last time point tested, compared with up to 5 and 3 days after Shuttle flights [5][3]. In the Twins Study, three inflammation markers, including C-reactive protein, rose up to eightfold in the first days after landing [11].

What crews do about it

• Staying healthy before launch. NASA’s Health Stabilization Program limits who can get close to crew before launch and uses medical screening, vaccinations and restrictions on crew activities to keep infections off the spacecraft [12].
• Vaccines: early signs are encouraging. In the NASA Twins Study, one astronaut’s flu vaccine given in space produced a normal immune response [13]. Because antibody-producing B cells stay stable in flight, NASA suggests vaccination may be an option against virus reactivation on future missions [4].
• Fitness may help. In 22 station crewmembers, those with higher aerobic fitness before flight had a 29% lower risk of virus reactivation, and those with better upper-body muscular endurance a 39% lower risk [14].
• Options under study. Candidate countermeasures include nutrition, exercise, medicines and vaccination against herpes viruses; none is yet a proven package [1].

After landing and recovery

• Most measures return toward normal after flight; in the Twins Study, the post-landing rise in inflammation markers was back to baseline when next measured, 36 days after landing [11].
• Virus shedding can outlast the flight. After long missions, chickenpox virus and cytomegalovirus were still found 30 days after landing, the last time point tested [5]. Researchers note this could pose a risk to newborn babies, adults who have never had these viruses and people with weakened immune systems [3].
• An Earth parallel: in Antarctic winter-over crews, a ground analog of long isolation, clinical shingles was about ten times more common than in the general population (33.3 versus 3.2 per 1,000 person-years) [4].

What it means for clinicians on Earth

These points are general education for clinicians, not advice about any individual patient. Coordinate with the person’s flight surgeon (the physician responsible for crew health) where there is one.

• Think of reactivated herpes viruses. Shingles-like rashes or other herpes-virus problems after a mission fit what is known about spaceflight [3][4]. Live chickenpox virus was found in saliva in 2 of 3 astronauts tested after a Shuttle flight [4].
• Consider vulnerable contacts. Shedding was still found 30 days after landing, the last time point tested [5]. Researchers note this could matter for newborns, people who are not immune and immunocompromised people [3]. This is a research observation, not a recommendation about contact. Follow standard infection-prevention practice and coordinate with the flight surgeon.
• Rashes may be stubborn. Rashes were the most common immune-related problem in flight [7], and there were 2 apparent treatment failures, both skin rashes, on the station [15].
• New allergy-like symptoms can appear in space even without a history of allergies [4].
• Short flights count too. Immune changes were measurable after a 3-day private orbital flight [8][9], so they are relevant for spaceflight participants as well as career astronauts.
• Vaccination history matters. Pre-launch vaccination is part of NASA crew health protection [12], and in one astronaut a flu vaccine given in space produced a normal response [13].