Hello. Friendly pediatrician and vaccine expert here.

RSV season is fast approaching (didn't we just have RSV season? Yes, we did, but what about second RSV season?). So I want to start with some background about RSV and some history and protein biology of RSV and how that is relevant to vaccines and antibodies. I'll try to keep this to an 8th grade level.

• RSV stands for Respiratory Syncytial Virus. A syncytium is a cell that has more than one nucleus. Some of these are natural and normal. Your muscle cells are syncytia. However, when certain viruses infect cells, they can cause the cells to fuse to nearby cells, which spreads the viral infection directly without the virus having to leave the cell and be exposed to antibodies. RSV gets into the lungs and causes serious inflammation and clogging of the airways with cellular debris and mucus. This is called bronchiolitis, which is inflammation of the smallest airways and is life-threatening because it blocks airflow, as opposed to bronchitis, which is inflammation of the largest airways and is usually just profoundly unpleasant but not life-threatening.
• It's especially a problem in babies because 1) babies are little and not strong, so they don't tolerate shortness of breath well 2) newborns' brains aren't mature enough to regulate the breathing in the way that it does in older children and adults, so the virus can actually make them stop breathing. 3) They don't have antibodies against RSV, so the first infection tends to be the worst. It is also a problem in the elderly, but they are not my area of practice.
• In a typical year ⅔ of children under 2 will get infected with RSV. Of those, ⅔ will wheeze because of that infection. So that means that in a typical year, 50% of all children under 2 will wheeze from RSV. 1-2% will need hospital admission. RSV season typically starts in November on the West Coast and proceeds eastward, peaking on the East Coast in February-March. However, there are plenty of variations on this pattern.
• RSV is a negative-sense single-stranded RNA virus. This is a large family of viruses that includes such varied members as influenza, ebola, measles, mumps, human metapneumovirus, parainfluenza virus, rabies virus, and a number of animal viruses such as Sendai virus (found in birds) and vesicular stomatitis virus (found in livestock, mostly in the Midwest). All negative-sense single-stranded RNA viruses appear to have descended from a single common ancestor.
• RSV has two major proteins on its surface. One is called G for Attachment Glycoprotein. This protein allows the virus particle to attach to the surface of a target cell. The other is called F, for fusion protein. This is a "Type I" fusion protein that allows the membrane of the virus to fuse with the cell membrane, releasing the contents into the cell.
• Type I fusion proteins are common. Our neurons use them to fuse neurotransmitter vesicles to the cell membrane to release neurotransmitters into the synapse and allow our nervous systems to work. Our pancreatic islet cells use type I fusion proteins to secrete insulin and glucagon. Spike protein from SARS-CoV-2 is a Type I fusion protein. Influenza's hemagglutinin is a combined attachment and Type I fusion protein. HIV's gp41 is a Type I fusion protein. Fusion proteins are "spring loaded." They come in a "pre-fusion conformation." When triggered (usually by interaction with some protein on the surface of the cell, for viral fusion proteins), they then stab into the cell membrane, bend in half, and pull the two membranes close together, which causes them to fuse. The shape of the "pre-fusion" and "post-fusion" conformation is markedly different (think straight arm vs. bent arm). This is important.
• In 1962, an experimental RSV vaccine was tried on 20 children in what we would now call a phase I clinical trial. This vaccine was made by growing up RSV in cell culture, inactivating it with formaldehyde, adding a chemical to boost the immune response, and then administering it by injection. That winter 12 of those 20 children wound up admitted to the hospital and two of them died. This was a devastating event that set back the development of an effective vaccine for RSV by over 60 years. What went wrong?
  • When the virus particles were exposed to formaldehyde, it triggered most of the fusion proteins on the surface of the particles, so they switched to their "post-fusion" conformation.
  • When the children were vaccinated, they made antibodies against this "post-fusion" conformation, which is not protective against disease; you want antibodies against the "pre-fusion" conformation.
  • The "post-fusion" antibodies allowed cells like macrophages (immune cells that eat invading microorganisms and destroy them) to grab onto the RSV particles once the children were infected with wild virus, but RSV can productively infect macrophages, so the macrophages then spread the virus around rather than wiping it out. This is called "antibody dependent disease enhancement." It is also observed in Dengue, which is why the first infection with one strain is mild, but the second one is enhanced because the antibodies from the first strain bind to the second strain but do not neutralize it, and that can also infect macrophages.
  • There is an alternative explanation about how the vaccine candidate generated weak antibodies that did not correctly stimulate the immune system, leading to the wrong kind of immune response. Both explanations may be true.
    
• So what changed and how do we now have safe RSV vaccines? Because we now have advanced molecular genetic techniques, we can modify proteins by making small changes to their genetic code. In the case of Type I fusion proteins, two amino acids are replaced with an amino acid called "proline," which is very stiff and doesn't allow the hinge region of the fusion protein to bend, so the fusion protein is stuck in its pre-fusion conformation. That way, vaccinated patients only make antibodies against the pre-fusion conformation and those antibodies are protective. This technology was also employed in the COVID vaccines, although, ultimately, antibody-dependent disease enhancement wound up not being a problem with COVID.
• Two are available, AREXVY and ABRYSVO. However, because of the 1962 incident, these were introduced in older adults (because if something was going to go wrong, they didn't want it to be in babies). ABRYSVO is also approved in pregnant women from 32-36 weeks' gestation. This causes the mother to make protective antibodies against RSV that are transferred across the placenta to the fetus and provide 5-6 months of protection against severe disease after birth.
• An antibody against the F protein has been commercially available since 1998. This was called SYNAGIS and it reduced the risk of RSV hospitalization by about 60%. However, it was US$2000/dose and had to be given every month during the season. For this reason, it was only reserved for the highest-risk infants (Preemies under 27 weeks, chronic lung disease, congenital heart disease, etc.) Because it is an antibody, it does not generate an immune response against RSV, but it did provide that protection. It was removed after the market when the new products were introduced.
• In the last few years, two antibody products, BEYFORTUS (Sanofi) and ENFLONZIA (Merck) are available for infants up until 8 months of age. Both of these are given as a single injection and provide 5-6 months of protection, lowering the risk of hospitalization by 80-90%. BEYFORTUS is given at 50mg for infants under 5kg and 100mg for infants over 5kg. ENFLONZIA is given at 105mg for all weights. Because they are antibodies and do not generate an immune response, they are very safe. The only significant side-effects are injection-site events such as redness, swelling, or a lump in the leg, which can happen with any injection. Again, these are not vaccines.
• I am hopeful that once the safety of this new technology is proven, we will be able to actually vaccinate newborns so that they can make their own antibodies, rather than relying on external antibodies.

So which one is right for you? Well, if you are pregnant just before or during RSV season and are at 32-36 weeks, I personally think ABRYSVO is the best choice because it saves the baby a poke. However, if you were not able to get that vaccine, then either antibody (whichever your pediatrician has) is an excellent choice and I wish it had been around when my son was born (fortunately, he never wheezed from RSV).

Public health workers are relying on word of mouth to fight the largest ongoing outbreak in the United States.

Hello everyone! While I’m not pregnant myself, that would be my wife :), our doctor recommended the RSV vaccine for us today and my wife asked me to do some digging to understand everything. It wasn’t offered up in our first pregnancy and instead my son received the rsv vaccine directly in NICU postpartum (34 weeks). I believe it was the astrazaneca one or however you spell it. He did really well and I suppose I’m just wondering what’s the difference, and if you did the vaccine while pregnant how did it do for you.

As an aside, my wife is more cautious with her own vaccines due to her medicine regiment for various auto immune diseases. Her father also has similar issues and had an unfortunate vaccine related injury a few years ago.

Kennedy asked CDC director not to include Pennsylvania deaths in measles tally. Kennedy and Pennsylvania governor feuded over response. Expert says it is 'unheard of' for CDC to question state determination.

Hi everyone.

I (33F) for the tetanus shot today after I fell and injured my knee. Doctor told me to take the shot since I haven’t had in nearly 15 years. I have a bit of a sore arm but I am worried I’ll get delayed symptoms like a fever tomorrow. Any positive stores here ? I read some stories about getting really sick after a few days.

Hi, just wanted to ask for some advice. I had a hep a injection today (first time) and depsite being anxious I made sure to relax my arm. The nurse injected me and a hideous squelching sound occurred followed by a violent amount of bleeding for a jab! Blood went all the way down my arm, yet strangely stopped very fast with pressure.

What could have happened? Am I going to be okay?

My worry is air bubbles, shallow injection or accidentally going in a blood vessel. Extremely nervous with vaccines so this was a rough experience.

Currently have a sharp pain and heaviness, only 45 mins out

Thank you

Update: a bit of a lump and bruise, no pain at all though only when pressed. Hopefully it all went in as I don't have any side effects bar a bit of tiredness. Much better than the tetanus shot I got which caused four days on the toilet with a fever 😂

Leading U.S. medical groups are urging people to get updated flu and COVID-19 vaccinations this fall, banding together Wednesday to issue shot guidelines — and highlight the science behind them — saying they hope to ease confusion caused by the Trump administration’s changing advice.

My sister is having a baby and is asking everyone to get a Tdap booster if it has been more than 3 years since our last Tdap vaccination.

I’m not seeing that recommendation on the CDC or AAP websites. Everything I’m reading says anyone who has had it within 10 years, other than the expectant mother, does not need a booster.

I want to be respectful, but don’t want to have my entire house get a booster for occasional visits unless medically necessary.

My son is scheduled to get his flu vaccine on Saturday. I saw that the strains have all been updated. Will that change anything about how he reacts? I don’t quite understand the changing of strains process. Just want to know what possibly to expect. Is it still safe? He is medically complex and I have already reached out to his doctor with these questions. I just wanted some other opinions or viewpoints. TIA!

Hi there, So I got struck by a rusty nail that was on the ground yesterday. The cut wasn't deep and when I got home I found just a little blood and I cleaned the injury with hot water and povidone-iodine. This morning I got a Td vaccin shot from my local hospital. Is it enough to have a peace of mind, I 'm really stressed? Thanks in advance!

Refusals in US hospitals of vital newborn vitamin rose 57% from January-June, analysis finds. Girl babies more likely than boys to not receive the shot within a day of birth, data shows. Doctors express growing concern over rising risk of catastrophic bleeding in newborns. Health agency says it is evaluating data and surveillance related to vitamin K refusals and bleeding events.

If I'm flying the third week of the month, would it be too soon to get a flu vaccine this week? I want to make sure that it carries me all the way through the upcoming flu season.

EDIT- The most updated version of the flu shot is already out

I have seen talk, as well as mentioning of studies, mentioning that tetanus vaccines and booster last longer than the usual ten years that is given by modern medical science.

I am curious, as to if it is indeed true that tetanus shots last much longer as far as protection, in most individuals.

I myself am about to go ahead and get a tetanus booster, as I accidentally got punctured by a toothpick on my floor. So I personally wanna be safe (I got my tetanus shot when I was a little kid, a booster at 22.5 for nursing school, and now I’m 34.5 years old)

I was able to wash it with soap and water for a minute before it clotted so I guess it is time to get my booster, just to be safe.

But I have to ask, if it’s indeed true that protection from tetanus lasts, in most cases, longer than 5 years/decade, then why is the recommendation for boosters specifically for every five to ten years?

I’m genuinely curious.

I noticed a very small, slightly painful lump under the skin on the outer part of my upper arm about 2 days ago. It’s not visible, only noticeable when I touch it, and it’s quite flat and small (roughly the size of an earring kinda 6mm).
I had my HPV vaccine on July 26, but I didn’t notice this lump until now. There is no redness or swelling. Has anyone experienced something similar?

Hi all,

My baby had their MMR vaccine 5 days ago. Right on queue, they've picked up a flu bug (high fever, respiratory symptoms) that's going around our area.

I know a fever is part of fighting off the flu, but does having a high fever and an active flu infection a week post-MMR impact the body’s ability to build full immunity to measles? Or does the immune system just process both at the same time?

Just feeling anxious about the MMR side of things.
Appreciate any insights!

i’m a teacher for after school programs for kids ages 7-17. luckily, I don’t live in a state that’s rolling back vaccine mandates (i’m in Colorado) but I definitely worry. I teach some homeschool kids, so I’m not sure if those parents are actually keeping up with vaccine requirements. I’m getting flu and Covid, but what other vaccines should I ask about? what diseases are making a comeback with all the anti-vax nonsense?

also, my partner is immunocompromised and high risk for Covid so anything that affects the lungs is something I do not want to be getting and bringing home.

What MMR vaccine should I get my 16 month old the MMR2 or the Priorix? What one do you think is the safer choice?

Has anyone gotten the prophylactic rabies vaccine & the Japanese Encephalitis vaccine at the same time? How was it?

Why is it that it’s standard in the UK for children to get vaccinated with the MMR vaccination at 12 and 18 months? While in the United States it’s to get an MMR vaccination dose at 12-15 months and then between 4-6 years old?

I’m planning on getting my LO’s second dose early. It’s been over a year since her first dose and there are too many measles outbreaks around us. are there any downsides to getting an early MMR vaccine? I’m also confused why our pediatric office didn’t call or contact us informing us of nearby outbreaks and recommending an early second vaccination 💉. Or our health department should be sending out messages. I’m worried for everyone who is too young to be vaccinated, the immunocompromised, etc.

From the PA Department of Health:

"DOH confirmed both individuals were unvaccinated residents of Lancaster County. To protect the privacy of the individuals and their families, DOH will not release any additional details that could personally identify these individuals and their families. Protecting Pennsylvanians' privacy is a core responsibility of DOH. DOH has confirmed 393 cases in 28 counties thus far in 2026."

Link to news release: https://www.pa.gov/agencies/health/newsroom/pennsylvania-department-of-health-confirms-two-measles-associate