It said a slight fever below about 38.5 degrees Celsius (101 degrees Fahrenheit) is part of the body’s normal response to infection and is not harmful. If it can be tolerated, rest and fluids are the ideal remedy, said English. A sustained temperature above 39 degrees Celsius is more serious, especially in infants. Taking aspirin, acetaminophen/paracetamol (such as Tylenol), or a non-steroidal anti-inflammatory drug such as ibuprofen to relieve pain and fever is not likely to impair the quality of the immune response.
Reactions usually result from the immune system’s response to the key component — an antigen that resembles whatever bug it is designed to fight.
Normally, when a body encounters a bacteria, virus or some other potential foe, immune defences seek to neutralise and destroy it. Chemicals that attract cells to kill the invader are released in a process that can raise the body temperature, Peter English, a consultant in communicable disease control in the UK, was quoted as saying by
Bloomberg. A vast army of so-called T cells and B cells are recruited to generate lasting “memory” of the foe and how to thwart it. “In learning to recognize the pathogen, the body goes through the same immune reactions as it would if it had met the pathogen for real, producing many of the same reactions,” English said.
Vaccines may also contain components that can induce a reaction, or enhance the immune response to the vaccine antigens, English said. Covid-19 vaccines may also include preservatives to prevent the vaccine from spoiling.
Regarding reactions being seen after the second dose, the report said it takes some time for the immune system to hone its response to a new pathogen. Immune memory cells are programmed such that when they encounter an invader a second time — either from a natural infection or vaccine antigens — they are primed to respond faster and more vigorously. That recognition typically triggers mass-production of immune-signaling molecules or “cytokines” that are responsible for the muscle aches, fevers, chills and fatigue recipients sometimes feel. The positive side of this is that the second encounter acts as a booster that should result in a more robust, longer-lasting immune response.
For those who have already survived the virus, their reaction to the shot may be more pronounced, but the benefits are likely to be so as well. Researchers have found antibody levels of those with pre-existing immunity were 10 to 45 times as high as those without at the same points in time after the first vaccine dose. Localised reactions to the vaccine occurred with equal frequency in both groups at the time of vaccination and resolved spontaneously days later. However, systemic side effects, like such as fatigue, headache, chills, fever and muscle aches, occurred after vaccination in 89% of those with pre-existing immunity compared with 46% of recipients without any.