The Food and Drug Administration approved and granted emergency use authorization on Aug. 22, 2024, for the newest, updated version of the COVID-19 vaccine. The Centers for Disease Control and Prevention quickly endorsed the new shot to protect against severe illness.

The 2024 summer wave of COVID-19 cases is a good reminder of why people need to stay up to date with their vaccines.

As the fall and winter seasons approach, the usual seasonal respiratory viruses, including flu and respiratory syncytial virus, or RSV, will also be on the rise. Vaccines are now available to help protect against these viruses.

The Conversation asked epidemiologist Annette Regan to explain why officials recommend that people get these shots over the coming months.

What strain is the new COVID-19 shot based on and why?

The COVID-19 vaccine has been updated several times since the original shot in 2020-21 to keep up with how the SARS-CoV-2 virus is changing.

In September 2023, the CDC recommended that all people get the newly updated vaccine that was designed to protect against XBB.1.5, the variant that had been dominant in the U.S. that summer, regardless of whether they had received all recommended shots in the past.

Now, in August 2024, the XBB.1.5 variant is no longer around and has been replaced by the KP.2 and KP.3 variants, which make up more than 60% of variants currently detected.

Moderna and Pfizer both made updated formulations of the COVID-19 shot that target this new KP.2 variant. It is a monovalent vaccine, which means it includes only the KP.2 strain.

This strain was selected because it was the most common variant at the time the choice was made. Even when new variants such as KP.3 emerge, we researchers expect the updated vaccine to protect against the newer strains. For example, the 2023-24 vaccine was designed to target the XBB.1.5 strain, and studies showed that it continued to protect against the JN.1 variant that later emerged.

The CDC recommends a single shot for everyone 6 months and older, with some exceptions. Children 6 months to 4 years old who have not received any prior shots of COVID-19 vaccine still need two or three shots of the updated 2024-25 vaccine. Adults 65 and older and children and adults with certain health conditions may require an additional shot as well.

People who recently had a SARS-CoV-2 infection may consider delaying their shot for three months after the illness, since risk of reinfection is thought to be low during the months after infection.

How did the 2023 updated vaccine perform?

Recent studies have shown that people who received the 2023-24 vaccine were 54% less likely to develop symptomatic COVID-19 illness, 39% to 51% less likely to visit an emergency department or urgent care with COVID-19, and 50% to 53% less likely to be hospitalized with COVID-19 compared with unvaccinated people.

The vaccine was most effective among those who had received their shot more recently.

What is the best timing for the shots?

COVID-19 hospitalizations and deaths have been rising since May 2024, with the highest rates seen in adults 65 and older and infants under 6 months old. Therefore, public health experts are recommending that people get the COVID-19 shot as soon as possible to protect against severe illness.

Because infants younger than 6 months are not old enough to be directly vaccinated, COVID-19 vaccination during pregnancy is the best way to protect these babies.

When it comes to the flu, cases and hospitalizations seem to rise steeply between November and December. Some years, however, such as the 2022-23 flu season, they start as early as October.

It’s important to remember that vaccines do not offer immediate protection. You need about two weeks for your body to generate enough antibodies to offer protection. September or early October is a good time to get the flu shot to ensure you are protected in time. However, if you aren’t able to get the shot before November, it’s important to know that it is still helpful to get the shot as long as flu is around.

Is it OK to get both the COVID-19 and flu shots at the same time?

Getting the COVID-19 and flu shots together can certainly make it easier to get up to date with these recommended vaccinations. Data shows that getting the flu shot and the COVID-19 shot together is safe and effective.

Some vaccine companies are working to develop a combined flu/COVID-19 vaccine to reduce the number of shots needed. This vaccine still needs to be approved by the Food and Drug Administration but could become available soon.

Who should get the RSV shot and when?

RSV is another common respiratory virus that can cause severe illness in young children and older adults. There are two groups of people who should get one of the three currently available RSV vaccines: adults 60 years and older and pregnant people. The CDC recommends a single dose of either GSK’s AREXVY, Moderna’s mRESVIA or Pfizer’s ABRYSVO for all adults 75 years and older and adults between the ages of 60 to 74 who are at increased risk of severe RSV disease.

Unlike the COVID-19 and flu shots, additional RSV doses are not currently recommended, because research suggests that the vaccine offers protection for at least two RSV seasons. These vaccines are relatively new, however, and more data will be needed to see how long this protection really lasts. Additional doses may be recommended in the future.

The CDC recommends only Pfizer’s shot for people in weeks 32 to 36 of pregnancy between September and January. The Pfizer vaccine is the only RSV vaccine that has been licensed and approved for use in pregnancy.

These vaccines are different from the antibodies that can be given to infants at or after birth, called Beyfortus (nirsevimab). The CDC recommends one dose of nirsevimab for infants who are younger than 8 months old born during or shortly before their first RSV season. For some children who are 8 to 19 months old and are at increased risk of severe RSV disease, a dose of nirsevimab may also be recommended during their second RSV season.

Both the RSV vaccine and nirsevimab offer safe and effective options for preventing RSV in young babies.

Pregnant people should get advice from their health care professionals about which option is best for them.

Annette Regan receives funding from the National Institutes for Health and the US Centers for Disease Control and Prevention. She has served on a Data Safety Monitoring Board for Moderna Inc.