If you have taken or will take the Covid-19 vaccine, don’t worry – there is no evidence that a lack of fever or headache means the vaccines did not work or will not work, experts say.

As recently reported in many media channels, the COVID-19 vaccines can cause temporary side effects. It’s a good sign, as it suggests your immune system is paying attention, preparing you in case of infection with the coronavirus.

But if you don’t experience side effects, does that mean the vaccine did not work?

The short answer is no, infectious-disease experts say. There is no evidence that a lack of side effects means the vaccinated person is unprotected against COVID-19.

The details require a bit of explanation, but the main reason physicians feel comfortable making that statement is simply math.

In the clinical trials, less than half of vaccine recipients reported moderate or severe episodes of “systemic” side effects such as fever, headache, and fatigue. Yet the drugs prevented most cases of the disease, according to those studies. So, by the process of elimination, some of that disease prevention must have occurred in the people with mild or no side effects.

Scientist and experts are saying that the vaccines still work even if you don’t have side effects. While many have side effects, many do not.

But just because this initial inflammation is successful in triggering the production of antibodies and other defences against disease, it does not necessarily cause a fever in the bargain.

And most other modern vaccines cause few, if any, side effects, yet they work well, said Dr Rall, co-author of Principles of Virology, a standard text in the field.

“We normally don’t see appreciable side effects following vaccination,” he said. (Among the exceptions, besides the vaccines for COVID-19, are those that protect against shingles.)

Whether people experience side effects might be determined by other factors besides their immune systems, including fatigue, stress, and how they perceive pain.

The rates of various side effects are slightly different in each of the COVID-19 vaccines, and in those that require two doses, they tend to be more common after the second dose.

With the vaccine made by Pfizer Inc. and BioNTech SE, for example, 3.7% of trial participants aged 18 to 55 reported fever after the first dose, and 15.8% reported fever after the second dose. Half of the recipients reported a headache after the second dose, but most were moderate or mild.

All these impacts are transient, generally lasting a day or two. And in rare cases of a serious allergic reaction called anaphylaxis within minutes after injection, physicians can readily treat that with an EpiPen or similar device containing epinephrine.

But given the alternative — a case of COVID-19 — it’s worth it.