If you notice you have swollen lymph nodes after receiving the COVID-19 vaccine, it might be a sign you have already had the virus. In a new study published on the preprint server medRxiv, which has not yet been peer-reviewed, researchers studied healthcare workers in the United Kingdom who received their first dose of the Pfizer vaccine. They found that four times as many with swollen lymph nodes after the vaccine had already suffered COVID-19 than those who were not previously infected. Other side effects such as fever, fatigue, muscle, and joint pain were also more common among those previously infected.