SINGAPORE - When healthcare student Melanie Hui, 21, got her first dose of the Pfizer vaccine in February, her arm was so sore she could not lift it to tie her hair.

Ms Hui, who is also a cross country runner, said she also felt fatigued and had headaches for about two days after the jab.

But when her grandmother, retiree Margaret Choong, 72, received her first dose of the Moderna vaccine last month, all she felt was a little pain at the injection site on her arm.

Ms Choong's two sisters, aged 76 and 62, who were vaccinated on the same day, were also generally well after the jab, although her elder sister felt a little tired.

Their experiences reflect the results of clinical trials for both the Pfizer and Moderna vaccines which showed that younger adults like Ms Hui tend to report more frequent and severe side effects than older people after their jabs.

In Pfizer's trials, those aged 18 to 55 reported experiencing side effects such as fever, fatigue, headaches and pain at the injection site more frequently than those aged 56 and above.

A similar trend was observed for the Moderna vaccine, with those aged 18 to 64 reporting side effects more frequently, compared to those aged 65 and above.

In both trials, side effects were also more commonly reported after the second dose.

For Ms Hui, the second dose of the Pfizer vaccine had caused her to develop a fever of 37.7 deg C, a splitting headache and nausea. She felt so tired she could not do anything for three days.

She said: "On the night after my second dose, I woke up every hour because I was so uncomfortable.

"Despite the side effects, I'm grateful and relieved I got to be vaccinated quite early on."

Her grandmother said: "I was most worried about my elder sister because she isn't in good health, but I was so happy and relieved that she was fine after the jab."

Associate Professor Hsu Li Yang, vice-dean of global health at the NUS Saw Swee Hock School of Public Health, said side effects tend to be more pronounced in those with more robust immune systems, such as young people and women.

He added: "Common side effects such as arm pain, muscle ache, fever, headaches and fatigue are the results of reactogenicity - the physical manifestation of our immune systems reacting to vaccination."

This does not mean the vaccine is less effective in those who do not experience side effects.

Prof Hsu said: "There is no correlation between absence or presence of these reactogenic side effects and vaccine efficacy. So, those with no side effects are just as likely to be protected by the vaccine."

Dr Ling Li Min, an infectious disease expert at Gleneagles Hospital, said: "Side effects such as fever and aches are a good sign that our immune response is working."

But she noted that vaccine effectiveness in the elderly has not been well studied so far. And that, theoretically, vaccines are somewhat less effective in older people, given the reduction in naive T cells available to respond to a vaccine as a person ages.

T cells are a type of white blood cell that work together with antibodies to eradicate the Sars-CoV-2 virus, which causes Covid-19.

But Dr Ling still encourages the elderly to get vaccinated.

She said: "Due to their ageing immune system, they are more likely to develop severe symptoms, end up in the intensive care unit, or die if infected."

Dr Ling emphasised that studies in Britain have shown that for people aged 80 and above, the Pfizer vaccine was 86 per cent effective in preventing symptomatic Covid-19 infection, more than 14 days after the second dose.

Full-time national serviceman Ho Lian Shun also had side effects after receiving his second dose of the Pfizer vaccine this month.

The 20-year-old developed a fever of 38 deg C, felt fatigued and sore, and was given a two-day medical certificate by the doctor.

General practitioners say they see a handful of patients each week for vaccine reactions, noting that younger people tend to report more pronounced side effects.

Dr Leong Choon Kit, a family physician at Mission Medical Clinic, said he sees about one patient a day for vaccine reactions. Most of these patients are educators in their 20s or 30s, and hardly any are elderly.

Dr John Cheng, head of primary care and family physician at Healthway Medical Group, sees the trend in both the patients who consulted him and vaccinated Healthway staff who are typically younger.

However, he said, other factors such as underlying medical conditions can also affect a person's response to the vaccine.