Top 20 Vaccines You Should Know About

Some Experts Say J&J Vaccine Recipients Should Get An MRNA Booster. Here's What We Know About Mixing Shots.

Some people who got the J&J vaccine are seeking mRNA booster shots.

The CDC and FDA don't recommend mixing vaccines, but some experts say it can't hurt.

Early data on mixing vaccines suggests it could trigger a stronger immune response.

As the Delta variant becomes the dominant COVID strain in the US, Americans are scrambling to strengthen their protections against the coronavirus.

For some people who got the single-dose Johnson & Johnson vaccine, that means getting another shot just in case.

However, the FDA and the CDC have not recommended doing so. Leading infectious disease expert Dr. Anthony Fauci said there's no data yet to support it.

"Even though individual physicians would be doing that, from a clinical trial basis there's no real fundamental scientific reason to do that right now," Fauci said when asked about J&J boosters in a White House press briefing. "When those data become available, you will see recommendations change."

J&J data suggests the shot is effective against variants

Public perception of the J&J vaccine has been marred by concerns about blood clots and manufacturing snafus. But from a practical standpoint, the vaccine works.

J&J has stood up against some worrisome variants, including the highly transmissible Delta variant.

The company found its vaccine produced a "strong" antibody response to the variants, according to two small, not-yet-peer-reviewed studies. It concluded that J&J vaccine recipients do not need a booster — and, for now, the CDC agrees.

"Right now, we have no information to suggest that you need a second shot after J&J, even with the Delta variant," CDC director Rochelle Walensky told Today.

Some people are getting extra shots anyway

Some people are still seeking boosters. Select public health experts are among those "topping off" their J&J vaccines with a dose of Pfizer or Moderna.

Angela Rasmussen, a virologist at the University of Saskatchewan, tweeted on Tuesday that she got a dose of the Pfizer vaccine to supplement the J&J vaccine she received in April. She said she was spurred by the Delta variant, and the history of successful vaccine boosters before COVID-19.

Other virus experts agree that, generally speaking, the strategy of mixing different kinds of shots to bolster disease protection can work well.

"You would expect some combinations to be: firstly safe, and to give — at minimum — comparable and quite possibly superior efficacy to some of the standard two-dose regimens," immunologist John Moore, a professor at Weill Cornell Medical College, told Insider.

Data from early studies about mixing vaccines have been promising. A recent study in the UK suggests that mixing the Pfizer and AstraZeneca vaccines yields a greater immune response than two doses of AstraZeneca's shot.

Another study in Spain found that mixing doses successfully produced a boosted immune response. However, the study has not yet been peer-reviewed, and those preliminary results were announced in an online presentation.

Related stories Getting a booster shot in the US may be logistically difficult

Twitter users who already had their minds made up about getting an mRNA shot to follow up their J&J dose had some logistical questions about how to get a shot when you're already, technically, vaccinated.

Dr. Vin Gupta, the COVID-19 chief medical officer of Amazon, tweeted an answer.

"If you received the 1-dose J&J, go and get 1-shot of Pfizer or Moderna as a 'booster' when you're able," he wrote. "Most I know who got J&J are doing it and are telling others the same — since two seems better than 1 re: delta."

Christopher Murray, director of the Institute for Health Metrics and Evaluation, told Insider he thinks following up with an mRNA shot will soon become "smart strategy" for those who got the J&J jab — especially teachers looking ahead to the next school year.

How 'mix And Match' COVID-19 Booster Shots Work, And Why We're Using Them

Fully vaccinated Americans will soon be offered extra doses of different COVID-19 vaccines.

This is known as a "mix and match" booster shot strategy.

Here's what we know about the benefits of mix and match booster shots so far.

Americans will soon be able to get so-called "mix and match" boosters — an extra COVID-19 shot that's different to the vaccine they originally received.

On Wednesday evening, the US Food and Drug Administration (FDA) authorized mix-and-match booster shots for people in the US who originally got Johnson & Johnson's single dose vaccine, and for older adults and vulnerable people who had Moderna's vaccine. 

Pending a final green light from the Centers for Disease Control and Prevention (CDC), eligible Americans vaccinated with Pfizer and Moderna's vaccines will soon be able to get a shot of any FDA-authorized vaccine booster after six months. People who got J&J's vaccine will be able to have one after two months.

This followed an influential US trial that suggested boosting with different vaccines was safe and effective.

Before, only immunocompromised patients, as well as vulnerable populations and older adults who'd had Pfizer vaccines, were eligible for boosters.

Mix-and-match boosters haven't been extensively tested in the real world — but there are lots of reasons why they're a good idea, experts say.

The reasons for the decision

In theory, different vaccines can stimulate the immune system in different ways, so giving people mix-and-match booster shots could produce a better immune response than an extra dose of the same vaccine.

"You'll reach a part of the immune system not reached as well by the initial vaccine," Dr. Robert Atmar, one of the principal investigators of the ongoing mix-and-match booster trial at Baylor College of Medicine, told Insider of the idea.

Some vaccines may be better at producing an antibody response, for example, while others might be better at stimulating white-blood cells called T cells and B cells to protect us. 

But this was "really a theoretical potential benefit" that's largely been seen "in the test tube," he said.

The main reason the US is allowing mixing and matching is, for now, practical rather than scientific. It's "really a public health question, in terms of ease of administration and distribution," Atmar said.

Some vaccines are simply more readily available than others — not every pharmacy or doctor's office carries all three vaccine brands.

Mix and match will help if one manufacturer has a problem, Pedro Piedra, professor of molecular virology, microbiology, and pediatrics at Baylor College of Medicine in Houston, told National Geographic.

The data on mix-and-match booster shots

In the UK, adults over 50 or at risk of severe COVID-19 have been able to get a mix-and-match booster since September 14. This decision was mostly based on early data from an ongoing UK-based trial, COV-Boost, which is not publicly available.

The National Institute for Health Research, which funded the study, said at the time that the trial showed mixed booster doses of COVID-19 vaccines given to fully vaccinated people provided a "substantial increase in vaccine-induced immune responses." 

The preference is for Brits to get Pfizer, regardless of what vaccine they had before, but a half-dose of Moderna is also available.

The US published its first landmark booster trial on October 13, which showed giving people who'd received J&J's one-shot vaccine a separate dose of any FDA-approved vaccine boosted their antibodies, without any serious side effects.

In the trial, a second dose of J&J's shot boosted people's antibodies four fold, while switching to a shot of Pfizer or Moderna boosted antibodies 35 and 75 fold, respectively. This could suggest mixing and matching provided more protection — but antibodies aren't a perfect measure of overall immune protection.

"The study really was not designed" for direct comparisons, Atmar said. Regardless of what a person received originally, getting one of the three booster shots "led to good antibody responses," he said.

Real-world results aren't yet known

But it's unclear what the true effect of mixing and matching will be because real-world data is scarce.

Dan Barouch, director of the Center for Virology and Vaccine Research at Boston's Beth Israel Deaconess Medical Center, told National Geographic that every combination should be specifically studied for safety and immune response. Even vaccines that use the same technology aren't identical, which could affect how a mix and match booster dose works, he said.

For example, Moderna and Pfizer's vaccines both use a genetic code to teach the body to make part of the virus to trigger an immune response, but they vary in multiple ways including dose, dosing interval, and the genetic sequence they use.

Atmar agrees you'd test all combinations "in a perfect world," but that it wasn't "feasible" in the US to, for example, study "boosters in persons who had gotten AstraZeneca, or who had gotten the Chinese vaccine, or the Sputnik vaccine."

Related stories Mix-and-match alone can't end pandemic

At the least, mixing and matching gives us options. In places where the same brand of booster isn't available, people can just take what's on offer.

But no matter how effective they are, the extra protection won't be enough to end the pandemic, vaccine experts say.

"The effect of a booster is much less than the effect of vaccinating unvaccinated individuals — and that means both here and abroad," Dr. Eric Rubin, who sits on the independent vaccine advisory committee to the FDA, said last week.

"If we're going to get out of this thing, we have to be vaccinating the unvaccinated," he said.

WHO Advisers Swap Out H3N2 Strains For Next Northern Hemisphere Flu Vaccines

The World Health Organization (WHO) today announced its advisory committee's recommendations on strains to include for the Northern Hemisphere's 2025-26 flu season, which swap out the H3N2 components but keep the current 2009 H1N1 and influenza B strains the same.

The three strains recommended for the trivalent vaccine are also the same as those recommended for the Southern Hemisphere's 2025 season vaccine, which the group weighed in on at its meetings in September 2024. 

Today's recommendations have separate H3N2 recommendations for egg-based and cell-based flu vaccines. Though the WHO recommends trivalent vaccines, some companies include a second influenza B strain targeting both lineages. The Yamagata lineage hasn't circulated since 2020, and the recommendation for that strain remains the same as for previous seasons.

H3N2 pick often a challenge

At a WHO briefing today, Ian Barr, PhD, deputy director of the WHO Collaborating Centre for Reference and Research on Influenza at the Doherty Institute at the University of Melbourne, said the H3N2 pick is always a challenge, because it changes more quickly than the other strains. "It's always the bane of our existence." He added that this season's H3N2 vaccine strain for the Northern Hemisphere didn't turn out to be a perfect match but has been a reasonable one. 

The severe flu season under way in the United States has come with a higher proportion of H3N2 activity than in other regions of the world such as Europe and China, where H1N1 has been predominant, Barr said. South America has seen a mix of H1N1 and influenza B, while Australia—like the United States—is experiencing a mix of H3N2 and H1N1.

Maria Van Kerkhove, PhD, WHO's director of epidemic and pandemic preparedness and prevention, said officials from the US Centers for Disease Control and Prevention (CDC) participated actively in this week's strain selection meeting and that the United States has been sharing genetic sequences from both people and animals. 

Country regulatory authorities and flu vaccine manufacturers take the WHO recommendations into account when starting the 6-month process for making the next season's flu vaccines.

The US Department of Health and Human Services (HHS) has canceled a March 13 Food and Drug Administration (FDA) vaccine advisory meeting to weigh in on the flu vaccine strain picks. However, HHS officials said the FDA would make its recommendations in time for manufacturers to update the vaccines for the next flu season. 

Two new picks for pandemic preparedness

In its twice-yearly flu vaccine strain consultations, the WHO advisers also comb through the latest zoonotic flu strains to see if any new candidate vaccine viruses are needed for pandemic preparedness.

Richard Webby, PhD, director of the WHO Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds and a researcher at St. Jude Children's Research Hospital, in Memphis, Tennessee, said the group recommended two new avian flu strains, one targeting an H5N1 clade 2.3.2.1a virus identified in Australia in a child who had returned from a trip to India. 

In its zoonotic flu candidate virus report, the group said the clade 2.3.2.1a viruses were detected in poultry in Bangladesh and in wild birds and poultry in India, where it also turned up in captive tigers, a captive leopard, and domestic cats. "The circulation of clade 2.3.2.1a viruses in these countries has continued despite the introduction of clade 2.3.4.4b viruses," the group wrote.

Webby said the second newly recommended candidate strain targets an H5N6 clade 2.3.4.4h virus once dominant in Southeast Asia that has reemerged in poultry in a few provinces in southeastern China. Two human H5N6 illnesses involving clade 2.3.4.4h were reported in 2024.

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