“In the United States, that’s hundreds of thousands, if not a million cases of flu that we can avoid each year,” said senior study author Anupam Jena, the Joseph P. Newhouse Professor of Health Care Policy in the Blavatnik Institute at HMS. “That’s a huge effect size.”

The findings provide additional support for the flu vaccine at a time when childhood vaccines have come under scrutiny in the United States. This January, the U.S. Centers for Disease Control and Prevention removed the annual influenza vaccine, as well as several others, from its childhood schedule of recommended vaccines. That change, which was widely condemned by medical societies and public health organizations, was blocked by a U.S. District Court in March.

“The federal government cited an absence of evidence that they want to see, and so we have provided that,” said Christopher Worsham, HMS assistant professor of medicine at Massachusetts General Hospital and first author on the study. “We have randomized data, and it shows that flu vaccines are effective for these young children.”

Sorting by birthday

Young children typically have an annual visit to the doctor scheduled around their birthday. For children born in the fall, those visits are a convenient time to get the flu vaccine. But children born in the summer will likely have appointments before the flu vaccine becomes available — they need an additional appointment to get vaccinated.

In previous research, Jena and Worsham found that the additional burden on caretakers results in lower flu vaccination rates in summer-born children. This creates a natural experiment, randomly sorting children into more- or less-vaccinated groups based on the happenstance of when they were born.

The researchers compared insurance claims data for summer-born and fall-born children between the ages of 2 and 5 over five flu seasons between 2016 and 2023. (They skipped the 2020-2021 and 2021-2022 seasons because of confounding factors from COVID-19.)

In each season, fall-born children were more likely to be vaccinated and less likely to catch the flu.

The vaccination rates for children with fall birthdays were between 8.6 and 12.5 percentage points higher than those with summer birthdays and the influenza diagnosis rates were 1.0 to 1.4 percentage points lower.

“Across these five seasons, we see that for every hundred kids who are randomly vaccinated because of when their birthday falls, somewhere between nine and 14 of them avoid a case of the flu that they otherwise would have caught,” said Jena, who is also a professor of medicine at Mass General.

For other illnesses that do not have vaccines, such as the common cold or gastrointestinal viruses, there was no difference in the infection rates of the two groups.

“It comes down to: vaccines work,” Worsham said.

More natural experiments

As children get older, birthdays and doctor’s appointments stop being so closely aligned. The researchers note that after about age five, the influenza diagnosis rates between fall- and summer-born children start to even out.

“The randomized data we have is limited to these very young children because their doctor appointments are tied to their birthday,” Worsham said.

That doesn’t mean that the vaccine isn’t effective in older children, teenagers, or adults — it definitely is, Worsham said. But after a certain age, birthdays are no longer good indicators of whether someone is more or less likely to have received the flu vaccine, so this experiment can’t capture vaccine effectiveness in older groups.

This work is just one example of a randomized experiment that can be found in existing data — there are many similar opportunities across different fields of medicine, the researchers said. Jena and Worsham are frequent collaborators and have co-authored a book, Random Acts of Medicine, on the subject.

“It is impossible to do a randomized controlled trial for every single thing that we want to know and understand,” Jena said. “But we have an incredible amount of data out there and there are randomized experiments like this sitting in that data, waiting to be uncovered.”

Authorship, funding, disclosures

Charles F. Bray, HMS hourly research assistant in health care policy and master’s student at Havard T.H. Chan School of Public Health, is co-author of the study.

This research was not directly supported by any funders.

The authors report no conflicts of interest related to this work. A full list of disclosures can be found in the paper.

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Catching influenza increases the short-term risk of cardiovascular conditions, and existing evidence has shown that the vaccine reduces the risk of heart attack and stroke by preventing flu infection in the first place. The new study, which included 1,221 adults aged 40 or older in Denmark, looked at whether cardiovascular risk was reduced even if people were infected with flu despite being vaccinated.

‘If confirmed by additional studies in other settings, this would strengthen the case for prioritising influenza vaccination among people at risk of heart disease or stroke and would support refining recommendations across Europe,’ says Dr Roberto Croci, Statens Serum Institut in Copenhagen.

The study used Danish health registry data from 2014 to 2025 and included individuals aged 40 and above with a first-ever hospital admission for a heart attack or stroke within a year after an influenza virus infection. It included all lab-confirmed flu virus infections that occurred during nine consecutive influenza seasons.

The study population comprised 660 males and 561 females aged 40 years and above, with a median age of 75. Most patients were hospitalised with a stroke (65%), while 35% had a heart attack.

The risk of a first-time hospitalisation for heart attack and stroke during the first week after testing positive for influenza was found to be significantly higher than for any other period before or after; it increased threefold for a stroke and fivefold for a heart attack. This increased risk was reduced by half for people who were infected but had been vaccinated against influenza for that influenza season.

The study did not account for differences in effectiveness between influenza vaccines, which can vary depending on how well the vaccine formulation matches the viral strains circulating in that season. It could also not assess whether vaccination timing or gender affected outcomes.

However, the results add to the growing importance of protecting against infectious diseases in people at risk of cardiovascular events. ‘Highlighting the dual protection offered by vaccination, against both infection and its cardiovascular complications, could have a substantial public health impact,’ the authors said.

Factoring the vaccine’s added protection against these conditions into economic and burden analysis could also help make a stronger economic case for influenza vaccination programmes.

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Every year, EMA issues EU recommendations for the composition of seasonal influenza vaccines on the basis of observations by the World Health Organization (WHO), which are informed by regular monitoring activities on the prevalence and characteristics of different influenza viruses worldwide.

Manufacturers of live-attenuated vaccines or egg-based vaccines should include these three virus strains for the 2026/2027 season:

• an A/Missouri/11/2025 (H1N1)pdm09-like virus;
• an A/Darwin/1454/2025 (H3N2)-like virus;
• a B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus.

Manufacturers of cell-based vaccines should include these three virus strains for the 2026/2027 season:

• an A/Missouri/11/2025 (H1N1)pdm09-like virus;
• an A/Darwin/1415/2025 (H3N2)-like virus;
• a B/Pennsylvania/14/2025 (B/Victoria lineage)-like virus.

Previously, most authorised influenza vaccines were formulated to protect against the four main strains of influenza responsible for seasonal flu, including the strain known as B/Yamagata.

However, since B/Yamagata has not been detected in circulation since March 2020, it is considered to no longer pose a threat to public health. As a result, there is no EU recommendation for a B/Yamagata strain in seasonal influenza vaccines for the 2026/2027 influenza season, which is in line with the respective WHO recommendation regarding the composition of vaccines for 2026/2027.
In case quadrivalent vaccines are still required (e.g. in regions where the transition to trivalent vaccines has not been finalised yet), vaccine manufacturers of inactivated vaccines can consider producing a quadrivalent vaccine containing two influenza B virus strains for the 2026/2027 season. For these vaccines it is proposed to follow WHO recommendations from previous years for the B/Yamagata strain, i.e. a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.

Influenza viruses continuously change and evolve. The periodic replacement of the virus strains contained in influenza vaccines is therefore necessary to keep the vaccines effective.

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“This is important given the rise in vaccine hesitancy, which has resulted in a downward trend in flu vaccination that coincided with a high rate of hospitalization this flu season,” said the study’s lead author, Shivan Mehta, MD, MBA, MSHP, associate chief innovation officer at Penn Medicine. “Many nudge interventions directed to patients only on vaccinations have shown limited effectiveness in the United States, so we wanted to make sure that we addressed both sides of the exam room: the patient and the clinician.”

The researchers believe these results might point to some strategies that could help boost how many people get the shot every year for an illness that has hospitalized up to 710,000 people each year since 2010—and killed as many as 52,000 Americans annually.

Nudging versus standard care 

The study tested several forms of “nudging,” a behavioral science concept that means small tweaks that make the healthiest choices the easiest ones. Patients who were eligible for the vaccine received flu shot reminder texts (or automated voice recordings), had automatic orders for a flu shot waiting for their clinician to approve, and monthly personalized messages were sent to providers that compared their patients’ vaccination rates to their clinician peers.

More than 52,000 people were randomly assigned to two groups: one that received all of the nudges or a “standard care” control group at either the University of Pennsylvania Health System or the University of Washington’s health system, UW Medicine The standard care team didn’t get any of the nudges and followed the usual path for getting a flu vaccine, which relies largely on the clinician remembering to offer the vaccine based on information in the electronic health records. Researchers found that almost 3,000 more people got flu shots when they were nudged than would have been expected if they got normal care.

Why nudging patients and clinicians worked 

Mehta and his colleagues are encouraged by their findings, driven mainly by the importance of communication and trust.

“We think the automatic order encouraged primary care physicians to have a conversation with their patients, and we know these clinicians still have a lot of trust from their communities,” said co-senior author Amol Navathe, MD, PhD, a professor of Medical Ethics and Health Policy, as well as a professor of Health Care Management in the Wharton School.

The work continues at scale 

The team has replicated their work at Lancaster General Health in the University of Pennsylvania Health System, which serves a rural and suburban patient base that looks somewhat different than the study populations in this study drawn largely from Philadelphia and Seattle. They are still analyzing the results of this replication study.

Since the work leverages existing tools in the electronic health record along with other available technologies but no additional staffing or human effort, it could be particularly attractive to health systems looking to augment their flu shot efforts.

“Future interventions could be more successful by complementing the automated communication with clinical staff to engage with patients that are still hesitant, and integrating flu vaccine nudges with other interventions focused on preventive health, like cancer screening,” Mehta said.

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Based on data from the World Health Organization (WHO), the European Medicines Agency (EMA) selected the three most common strains of flu – a break from previous years in which four strains were targeted.

Influenza viruses continuously change and evolve. The periodic replacement of the virus strains contained in influenza vaccines is necessary to keep the vaccines effective. Every year, EMA issues recommendations for the composition of seasonal influenza vaccines based on trends seen last winter and monitoring data from the southern hemisphere flu season.

For 2024/2025, the Agency is recommending a transition from quadrivalent (four-strain) to trivalent (three-strain) vaccines that do not include the B/Yamagata component which was included last year. The B/Yamagata strain of the influenza B virus has not been detected in circulation since March 2020. This is thought to be due in part to the public health measures put in place to limit the spread of COVID-19 during the pandemic. Influenza B viruses are responsible for a quarter of annual influenza infections.

Manufacturers of live-attenuated vaccines, or egg-based trivalent vaccines should include these three virus strains for the 2024/2025 season:

• an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
• an A/Thailand/8/2022 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

Manufacturers of cell-based trivalent vaccines should include these three virus strains for the 2024/2025 season:

• an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
• an A/Massachusetts/18/2022 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

The EMA said manufacturers of inactivated vaccines can consider producing a quadrivalent vaccine containing two influenza B virus strains for the 2024/2025 season. ‘In that case a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus in addition to the strains mentioned above is considered appropriate,’ the Agency said in a statement.

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Influenza viruses continuously change and evolve. That is why it is necessary to review the virus strains contained in influenza vaccines and, if necessary, update them before the new flu season.

Every year, the European Medicines Agency (EMA) issues recommendations for the composition of seasonal influenza vaccines well in advance of flu season, giving manufacturers time to begin production. Some years, there is not change; other years, one or two of the strains are replaced.

The regulator advises companies which strains of flu viruses they should protect against when formulating the revamped vaccine. This is based on WHO data which draws on the experience of the southern hemisphere, where flu season is just beginning.

Based on this data, the EMA’s Emergency Task Force recommends moving from a four-strain vaccine to a three-strain vaccine which does not include the B/Yamagata component featured in recent years. This is because that flu virus has not been very common recent flu seasons, north or south.

In fact, it has not been detected since March 2020, when COVID-19 was declared a global pandemic. This is thought to be due in part to the public health measures put in place to limit the spread of COVID. Influenza B viruses are responsible for a quarter of annual influenza infections.

Given that the B-Yamagata virus strain no longer seems to pose a threat to public health, it is not necessary to include it in the formulation of influenza vaccines, the EMA said.

The ETF recommends that this strain should ideally be removed from all live-attenuated vaccines from the 2024/2025 season. In the interest of guaranteeing vaccine supplies for the coming vaccination campaign, the transition to a trivalent composition for all other influenza vaccines should be completed for the 2025/2026 season.

On foot of this, the following strains are now the priority for the 2024/2025 season.

Manufacturers of live-attenuated vaccines, or egg-based trivalent vaccines should include:

• an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
• an A/Thailand/8/2022 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

Manufacturers of cell-based trivalent vaccines should include:

• an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
• an A/Massachusetts/18/2022 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

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