13 of the most common monkeypox vaccine side effects, from a 'bleb' to a hard lump

• The US is switching to a skin-deep monkeypox vaccine strategy, to save on supplies.
• Using this new technique, there should be "enough" shots available to vaccinate everyone at risk, federal officials say.

More and more people are getting vaccinated against monkeypox — and becoming acquainted with the common side effect: a hard lump.

"I've had a lot of patients show me their lumps," Elsbet Servay, clinical director of immunizations at Callen-Lorde Community Health Center in New York City, told Insider. "It's a normal, expected reaction."

This swelling is one of the 13 most common side effects healthcare workers typically see after administering Jynneos (the two-dose smallpox vaccine that also protects against monkeypox).

It's nothing to worry about, Servay said: Lumps and aches are "a part of your immune system doing its job."

If you are part of the select at-risk group advised to get vaccinated against monkeypox, here's what you need to know.

There are 2 ways to administer the shot

The US is dishing out monkeypox shots at a faster clip these days — thanks to both increased vaccine production, and a new skin-deep dose-saving technique. Federal officials said Tuesday that there should be "enough" vaccine supply now to inoculate everyone "in the at-risk community."

Instead of injecting a full dose into the fatty area at the back of a person's arm (a "subcutaneous injection"), some healthcare workers are following CDC advice to inject a fifth of the dose just under the surface of the skin (an "intradermal injection") at a very tight 5 to 15-degree angle.

"The technique is a bit of a dying art," Servay told Insider. "It's just a little trickier to administer, but we do expect it to offer protection."

If you get an intradermal injection, you want to see a 'bleb'

Because the intradermal injection is delivered so shallowly, you can actually see the vaccine liquid appear immediately inside the body, just under the surface of the skin:

If the intradermal injection does not immediately result in this bubble of liquid under the skin (often referred to as a "bleb" or "wheal"), the injection has failed, and has to be re-done.

Callen-Lorde has trained eight nurses in intradermal vaccine technique for monkeypox, done with special needles that can measure "a very small amount of liquid" with "very fine syringes," Servay said.

But, aside from this fine-tuned procedure, the effects of the monkeypox vaccine, whether given intradermally or subcutaneously, should be quite similar.

The most common side effects are firmness, itchiness, and redness

"The most common vaccine side effects are pain, redness, swelling, hardness, itching at the injection site," Servay said. These side effects may be slightly more pronounced with intradermal injections than they were with subcutaneous shots, in large part because "under the skin there's a more evolved, richer immune system" than in arm fat.

"When you think of all the cuts and scrapes that people get in day to day life, it makes sense," Servay added.

According to a 2015 study of more than 350 vaccine recipients, side effects are more common among people who get Jynneos intradermally than those who receive the bigger, deeper subcutaneous shots. But, there is one side effect that is far more common among people who get the vaccine injected into their arm fat.

The tenderness and pain at the injection site after a subcutaneous shot into the back of the arm can be worse than the pain after an intradermal forearm injection, which tends to just be more itchy and red.

Dr. Graham Walker, an ER physician in San Francisco told Insider that his injection site was "definitely tender" for a while after his first subcutaneous shot a few weeks ago.

"One time I bumped the back of my arm on something and it hurt a LOT for a few seconds, like a stubbed toe," he said.

Lumps and bumps can linger for days or weeks but are 'normal' and 'expected'

The most common shared complaint among people who've gotten the Jynneos vaccine — whether their injection was administered intradermally or subcutaneously — has been the presence of a "bump," "knot," or hard "lump" lingering for days or weeks afterwards. (That watery bleb, on the other hand, starts to dissipate almost immediately, and shouldn't be noticeable by the next day.)

One microbiology student described his first (subcutaneous) shot lump on Twitter as the size of a "nickel" but said his second (intradermal) injection resulted in a hump as big as an "egg."

Second dose side effects can often be more pronounced than first doses, because the immune system has already been primed to react. (Be aware that some health departments, including New York City's, are not scheduling second doses just yet.)

It takes 2 shots + 2 weeks to be considered 'fully vaccinated'

Servay said if there's one word of caution she would offer to people getting monkeypox vaccines, it's that the vaccine "doesn't work right away" and people "really don't have any" protection in the first days after their shot is administered.

"I have seen cases of monkeypox in people who probably acquired it slightly before or around the time they got their first dose," she said.

An immune response to monkeypox starts ramping up at about two weeks after a first shot, but "two weeks after that second dose is when you get maximum protection," Dr. Demetre Daskalakis, the US's deputy monkeypox response coordinator, said during the briefing Tuesday.

"That shot is not for today," he said, stressing there are "lots of other strategies" to reduce the spread of the virus in the meantime.

However, if you're one of the more than 18,900 people nationwide who've recently had monkeypox, there's no reason to rush out and get shots. Like smallpox, immunity from a prior monkeypox infection should last for a while, experts say.