Dr. Benjamin Singer, pulmonary and critical care medicine specialist at Northwestern Medicine, and Dr. Wayne Giles, dean of the school of public health at the University of Illinois at Chicago, provide answers to commonly asked questions about the COVID-19 vaccine. Responses were edited for length and clarity. 

What are the most common side effects of the vaccine?

Giles: Some side effects include arm swelling, fever, aches and pains. Those are relatively short and usually last between 24 and 36 hours. 

Singer:  The side effects are generally manageable and go away on their own. None of them are life-threatening except for very rare allergic reactions, which are quite uncommon.

What is the time frame for reaching immunity after receiving the vaccine?

Singer: A low level of immunity is attained within a few weeks after the first dose, but the very high levels from the trials — that 95 percent of protection — are attained  about two weeks after the second shot.

What precautions still need to be taken once someone is vaccinated? 

Singer: Some of the open questions are, “Even if you are protected from the virus, can you still carry it in your nose and spread it to other people?”  That seems very unlikely, but it is possible. We don’t actually know that yet. Until we know more and while COVID-19 is so prevalent in the community, even after being vaccinated, you should wear a mask and follow all the same procedures when you’re outside of your house. 

What is herd immunity and how does it relate to COVID-19? 

Singer: Herd immunity is when we reach a level of immunity in the population such that if someone is infected, they only ever encounter immune people, so the virus can’t spread. Herd immunity, for any virus, has never been achieved through natural immunity or natural infection. It can really only be achieved through high-level, high-efficacy vaccination. It’s still a bit unclear about what percent of the population needs to be immune in order to achieve that level of herd immunity, but it’s upwards of 70 percent. 

What age groups are currently able to receive the vaccine and when will younger people be eligible?

Giles: The Pfizer vaccine is approved for people ages 16 and above, and Moderna is for ages 18 and above. Trials looking at younger age groups are under way as well. I would hope that by summer 2021, those additional trials will be able to be reviewed. Pfizer is conducting a trial of ages 12 to 16. Depending on the results from that and other trials, it could be months to a year before younger individuals become eligible for vaccination.