Does a flu or COVID booster shot show up on your wearable's heart rate and HRV?

This is a different question from whether your wearable can catch an infection before you feel sick — that's about a disease process your body hasn't been told about in advance. A vaccine is the opposite: a known, scheduled, intentional immune challenge, which makes it one of the cleanest natural experiments in "what does a normal immune response actually look like on a chart."

Why a vaccine moves your numbers at all

A vaccine works by getting your immune system to react to something it doesn't recognize, and that reaction starts within hours — well before any muscle soreness or fatigue sets in. Early in that process, immune cells release signaling proteins (cytokines) that nudge the autonomic nervous system toward a more sympathetic, "on alert" state. That's the same physiological lever that gets pulled by a mild infection, a fever, or acute psychological stress, and it shows up on a wearable the same general way:

• Resting heart rate rises — mild systemic inflammation increases metabolic demand and sympathetic drive.
• HRV falls — reduced parasympathetic activity relative to sympathetic tone, the standard autonomic signature of the body dealing with a stressor.
• Sleep gets lighter and more fragmented — the WHOOP study specifically found an elevated share of light sleep (and less deep/REM) the first night.

None of this means anything is wrong. It's the measurable footprint of the immune system doing exactly what the vaccine asked it to do.

How much it actually moves, and for how long

These are study averages, not guarantees for any one person — the underlying research found wide individual variation, and the Garmin team specifically noted that some people showed clear heart-rate and stress-score changes despite reporting no side effects at all on a symptom questionnaire. A flat reading after your shot is just as normal as a bumpy one.

Why the second mRNA dose hits harder, but the first viral-vector dose does

This isn't random — it tracks the biology of each vaccine type. mRNA vaccines teach your immune system to recognize a target protein, and the second exposure to that target triggers a faster, larger, "memory" response from an immune system that's already primed — which is also, not coincidentally, part of why they work. Viral-vector vaccines (like AstraZeneca) use a harmless carrier virus to deliver their payload, and that carrier itself provokes a noticeable innate immune reaction the first time your body meets it; by the second dose, your immune system has often built some resistance to the carrier itself, blunting the reaction. Same broad mechanism — cytokine-driven sympathetic activation — but the timing of the bigger bump differs by platform.

Why flu shots don't show up the same way

Standard inactivated seasonal flu vaccines are a much smaller immunological event than an mRNA or viral-vector COVID vaccine — a lower perceived "threat" for your innate immune system to respond to, and a much longer track record of mild local-only reactions in most people. The Garmin-based study that quantified a clear COVID-booster effect in the same population found no measurable heart-rate or stress-score change after influenza vaccination. If you've had both and felt more after the COVID shot, your wearable data likely backs up that impression — it isn't just in your head.

Can your pre-vaccine baseline predict how rough it'll be?

There's an early hint that it might, though the evidence is preliminary. A 2024 study using the same Garmin device in over 2,000 people tried predicting which users would report worse side effects using only wearable data collected before the shot — resting heart rate, heart rate, and HRV in the days leading up to vaccination. The models worked better than chance but only modestly (reported accuracy scores in the 0.69-0.74 range on a 0.5-to-1.0 scale, where 0.5 is a coin flip and 1.0 is perfect). That's an interesting research direction, not something you can act on individually — it doesn't mean a slightly lower HRV reading two days before a shot tells you anything reliable about how you personally will react.

Telling a vaccine reaction apart from something else

Because the mechanism (cytokine-driven sympathetic shift) overlaps with early illness and overtraining, use timing and a known trigger to separate them:

• A vaccine reaction starts within hours of a shot you know you got, peaks around 18-24 hours later, and is essentially gone by day 3-4. You can date it precisely because you know exactly when it started.
• Early illness has no known trigger event — your baseline suddenly drifts for no reason you can point to, and if it's a genuine infection it tends to keep moving in the wrong direction rather than resolving in a few days.
• Overtraining or a hard training block correlates with a training-load spike you can also see in your own data, not an injection.

A simple way to work through it in practice:

1. Note the exact date and time of the shot. Everything that follows is measured against this.
2. Expect a dip the first night, not before. A change that predates the injection isn't a vaccine effect by definition.
3. Check day 3-4. If resting heart rate and HRV are back within your normal day-to-day range, that's consistent with a typical vaccine reaction closing out.
4. If day 5 arrives and nothing has moved back, stop treating it as the vaccine and start troubleshooting it as you would any other unexplained baseline shift — a new illness, a bad night, dehydration, alcohol, or a training load change are all more likely explanations by that point than a lingering vaccine effect.

If you track your recovery score across multiple sources — say Apple Health sleep data alongside a WHOOP or Apple Watch heart-rate stream — pulling up the same few nights side by side makes it easier to see that the dip is confined to a short, dateable window rather than an open-ended slide. If something still looks off once that window should have closed, that's a reasonable moment to ask Vita to help you look at the pattern across your sources rather than guessing from one number.

When to stop assuming it's "just the vaccine"

The reactogenicity pattern described above is mild, short, and self-limited — a bump in resting heart rate, a dip in HRV, maybe a rough night of sleep, gone within a few days. That is not the same thing as new or ongoing chest pain or pressure, a racing or pounding heartbeat that doesn't settle, fainting, or shortness of breath, particularly if it starts a few days after an mRNA vaccine dose rather than fading. Rare cases of myocarditis or pericarditis following mRNA COVID-19 vaccination have been documented, more often in younger males and more often after a second dose, and they present with exactly those symptoms rather than as a quiet shift in overnight metrics. A wearable chart is not a diagnostic tool either way — if you notice symptoms like these, seek medical care promptly rather than waiting to see if your recovery score recovers on its own.

The takeaway

A next-day bump in resting heart rate and dip in HRV after a COVID vaccine — especially a second mRNA dose or a booster — is one of the most consistently documented "expected" disruptions in wearable data, visible across tens of thousands of users in more than one independent study. It typically peaks around 18-24 hours after the shot and clears within three to four days. A flu shot usually produces little to no measurable change by comparison. If your data looks completely normal after a vaccine, that's also normal — plenty of people show no detectable shift at all. The one pattern that doesn't fit this description is new chest symptoms that start rather than fade in the days after — that calls for medical attention, not a wait-and-see approach to your recovery score.