Health experts say the extra dose will help to strengthen the body's defenses against the highly contagious delta and omicron variants. People who are immunocompromised may need an additional dose to help them generate a robust immune response more in line with that of people with healthy immune systems.

The Da Nang Centre for Disease Control (CDC) said that as with the first round of vaccines, some people may experience a few mild to moderate flu-like symptoms after the shot.

Here’s what we know about the side effects from the boosters:

Side effects after the third dose are also likely to vary from person to person, with some people experiencing minimal discomfort. The side effects will be similar to those of the first two doses. The most common side effects are pain at the injection site, fatigue, headache, muscle and joint pain, and chills. While rare, cases of myocarditis or inflammation of the heart muscle and pericarditis in the sac that surrounds the heart have been reported in adolescents and young adults who have had the Pfizer or Moderna vaccines.

Swollen lymph nodes can be a normal and expected side effect after receiving the COVID-19 vaccine. Some people can see enlarged lymph nodes under the armpit, along the collar bone and even up into the neck. This natural response can occur on the side where an individual received their injection and was a recognized side effect in both the Moderna and Pfizer-BioNtech vaccine recipients. Enlarged lymph nodes are common after the COVID-19 vaccine and are your body’s normal reaction to the vaccine. In most cases, no additional testing or no medical intervention is needed for swollen lymph nodes after recent vaccinations unless the swelling persists or if the individual has other health issues. These symptoms should generally go away within six weeks.

However, if you feel a lump in your armpit that lasts for more than six weeks, develop prolonged fever, and suffer unintentional weight loss after your vaccination, you should go to hospital for a check-up.

The Da Nang CDC recommends that people should monitor their health after vaccination, as well as should seek medical attention immediately if they experience any of the following symptoms: numbness in and around their mouth, rashes, cyanosis, skin redness, bleeding into the skin, itchy throat, tightness in the throat, difficulty speaking, severe headaches, lethargy, confusion, coma, convulsions, chest pain, shortness of breath, feelings of having a fast-beating, fluttering or pounding heart, fainting, vomiting, diarrhea, stridor, cyanosis, dizziness, abnormal fatigue, abdominal pain, and persistent fever.

The Da Nang CDC recommends drinking plenty of water prior to getting a vaccine and after vaccination in order to manage the side effects better, as well as taking an over-the-counter pain reliever after the vaccine to help manage any discomfort if needed.

Reporting by THANH TINH – Translating by H.L