According to the World Health Organization, about 180 COVID-19 vaccines are being produced around the world, and many use slightly different methods to fight the virus. The next-generation mRNA technology employed by Pfizer and Moderna use a fragment of genetic material, in this case -RNA, which encodes a protein known as ‘spike protein’ also present in Covid-19. On entering our body, this RNA instructs our cells to produce the spike protein which generates an immune response from our body. Johnson & Johnson, AstraZeneca and Sputnik-V also make use of the spike protein to trigger an immune response but use a weakened viral vector like the adenovirus (flu virus) to introduce the protein. Bharat Biotech’s Covaxin uses an inactivated strain of Coronavirus acquired from a patient who suffered an asymptomatic case of Covid-19. These viruses are inactivated using heat/chemical treatments and thus cannot replicate as opposed to live-attenuated viruses which have been weakened so can still replicate and stimulate an excellent immune response. A subunit vaccine has purified virus proteins that serve as antigens.
As a side effect of the vaccine, people have had one or a combination of temporary symptoms such as fatigue, body aches and pains, chills, a mild fever, or just a very sore, tender arm. To understand why some vaccines may have side effects, we must first understand how our immune system works. When an antigen, in this case the vaccine, enters the body, it triggers a primary immune response. This involves the recognition of the antigen as ‘foreign’ resulting in the production of antibodies by B-cells. These antibodies are specific to the antigen and they bind to the surface of these antigens which help the T-killer cells to identify the antigen and destroy them. Another type of T-cells known as the memory cells remember the antigen for future encounters. So when the same antigen re-enters the body it triggers a secondary immune response which is what happens in the case of vaccines that require a second dose. Since the T-cells remember the antigens, the immune response is much stronger, which is why the side effects might be much higher after the second dose. All these immune cells produce inflammatory proteins known as cytokines, which are chemical messengers that help coordinate the immune response and also trigger a fever which is a common side effect of the COVID-19 vaccines. A higher temperature makes the body less hospitable for the virus, and the rise in temperature stimulates the body to create more immune cells. These inflammatory chemicals can also cause muscle pain, fatigue, headaches and other symptoms.
Relatively uncommon side effects include a rash at the site of the vaccine, known as ‘Covid arm’ and the extremely rare cases of Blood clotting which is responsible for the pause on the J&J vaccine. Cases of similar blood clotting were also observed as a side effect of the AstraZeneca vaccine, the common link between the two being that they’re both adenovirus vaccines. The patients with clots also had low numbers of platelets which are in fact required for the blood clotting process. Normally, a person with a low platelet count is unable to form clots and as a result bleeds. A possible explanation for this observation is that an immune reaction as a result of the vaccine may have set off uncontrolled clotting that bound up all their platelets. Anaphylaxis, which is a very severe allergic reaction, is another very rare side effect of certain vaccines.
As observed, women and younger people tend to report a greater intensity of side effects than older and male individuals. In clinical trials, the CDC found that side effects were more common in people under the age of 55.
It’s possible that younger people have better immune systems, which explains why they have more symptoms. Another theory suggests that older people had more time to be exposed to different coronaviruses that are similar to COVID-19. So taking the shot won’t be as triggering to an older person’s immune system compared to someone who is younger. As to why women have been experiencing more severe side effects is still unknown. It is possible that women might simply be more likely to report adverse effects after a vaccine, based on reports that women are more likely than men to seek medical attention. But this is just an assumption. Experts suspect that in women, particularly premenopausal women, the levels of estrogen help activate the immune response to illness and, therefore, to vaccines. Estrogen causes more antibodies to be created by B cells, which might be a possible explanation to the stronger immune response and thus more side effects.
On the other hand, if one doesn’t experience any side effects doesn’t mean that the vaccine is not effective or there’s no immune response. Everyone has different levels of reactogenicity.