No reaction to COVID-19 vaccine? No problem – it’s still working

You’ve heard about the side effects from getting the COVID-19 vaccine. But after you got your shot, you didn’t have a reaction – nothing at all. Should you be concerned? Is the vaccine working?

These are questions some patients have asked me after getting their shot. Fortunately, there’s no reason to be worried. Just because you didn’t have a reaction – or not much of one – doesn’t mean your body isn’t mounting a response to the vaccine. The reality is that not everyone has a reaction. As a matter of fact, studies show only about 50% of patients experience side effects. The most common ones are sore arms, fever, chills and fatigue.

How does the vaccine work?

For the Pfizer and Moderna vaccines, mRNA (or messenger RNA) is injected, sending specific instructions for your body to create a piece of the SARS-CoV-2 virus, the virus that causes COVID-19, called the spike protein. Once the spike protein is created, your body’s immune system recognizes this protein as a “foreign invader” and starts to attack. This is when your immune system is stimulated and antibodies produced. But you don’t actually get sick because your body is only exposed to a small portion of the virus and not the intact virus that can cause the disease. The antibodies against the spike protein remain in your body and are available and ready to protect you if you’re exposed to the SARS-CoV-2 virus. With the Johnson and Johnson vaccine, the instructions for making the spike protein are delivered to your cells in the form of DNA (as opposed to mRNA). A type of weakened cold virus (called an adenovirus vector) transports the DNA into your cells.

Why might there not be a reaction?

It’s not entirely clear why some people experience side effects and others don’t. It’s likely related to variations in the immune system and its response, which may be influenced by things like age, genetics or underlying medical conditions. We’re also seeing that women and younger people are more likely to experience reactions after the vaccine. This may be because they tend to mount a more robust immune/inflammatory response but the reasons for this aren’t clear. The immune system is very complex, making it hard to pinpoint exactly what makes someone react or not.

Can medication affect side effects?

Because reactions after the vaccine are related to an immune response/inflammation, there’s concern that taking anti-inflammatory medications (such as ibuprofen) can disrupt the body's natural response to the vaccine. The U.S. Centers for Disease Control and Prevention recommends against taking these anti-inflammatory medications pre-emptively before vaccination. However, it’s OK to use them after the shot if you’re experiencing discomfort.

The bottom line is not to worry if you had a strong reaction or not. Studies show a high level of protection for everyone vaccinated – whether there was a strong reaction or none at all.