Ciena Set to Join S&P 500; Arrowhead Pharmaceuticals to Join S&P MidCap 400; ADT and OneSpaWorld Holdings to Join S&P SmallCap 600

Rhea-AI Summary

ADT (NYSE: ADT) will join the S&P SmallCap 600 effective prior to the opening of trading on Feb 9, 2026, as part of index changes by S&P Dow Jones Indices. The rebalancing also adds Ciena (CIEN) to the S&P 500 and moves Arrowhead Pharmaceuticals (ARWR) into the S&P MidCap 400 on Feb 9; OneSpaWorld (OSW) joins the SmallCap 600 on Feb 10.

Dayforce was removed following an acquisition by Thoma Bravo, and Dynavax was deleted ahead of its pending acquisition by Sanofi.

Positive

• Ciena added to S&P 500 on Feb 9, 2026
• ADT added to S&P SmallCap 600 on Feb 9, 2026
• OneSpaWorld added to S&P SmallCap 600 on Feb 10, 2026
• Arrowhead moved into S&P MidCap 400 on Feb 9, 2026

Negative

• Dayforce removed from S&P 500 after Thoma Bravo acquisition
• Arrowhead removed from S&P SmallCap 600 on Feb 9, 2026
• Dynavax removed from S&P SmallCap 600 ahead of pending Sanofi deal

News Market Reaction – ARWR

-5.02%

-5.02% Session close to close

In the Feb 5 session, ARWR declined 5.02%, reflecting a notable negative market reaction.

Data tracked by StockTitan Argus on the day of publication.

Market Context

The stock moved -5.0% in the session following this news. A negative reaction despite the index recl...

Analysis

The stock moved -5.0% in the session following this news. A negative reaction despite the index reclassification would fit a pattern where strong fundamental or strategic updates sometimes met muted or even adverse price responses, as seen after robust fiscal 2026 Q1 results. Investors have also recently digested sizable equity and convertible offerings plus insider sales, which can influence sentiment. In that context, index changes may be overshadowed by concerns around dilution or capital structure.

Key Figures

Effective date: February 9, 2026 Effective date: February 10, 2026 Index additions: 4 additions listed +2 more

5 metrics

Effective date February 9, 2026 Index changes including ARWR addition to S&P MidCap 400

Effective date February 10, 2026 Further S&P SmallCap 600 changes involving OSW and DVAX

Index additions 4 additions listed Ciena, Arrowhead Pharmaceuticals, ADT, OneSpaWorld Holdings

Index deletions 4 deletions listed Dayforce, Ciena, Arrowhead Pharmaceuticals, Dynavax Technologies

First index year 1884 Year Charles Dow invented the first index

Historical Context

5 past events · Latest: 2026-02-24 (Neutral)

Pattern 5 events

Date	Event	Sentiment	24h Move	Catalyst
2026-02-24	Investor events update	Neutral	+0.8%	Announced participation in multiple March 2026 investor conferences and meetings.
2026-02-05	Earnings and approval	Positive	-0.2%	Reported strong Q1 results with REDEMPLO approvals and multiple new trials.
2026-01-27	Clinical trial start	Positive	+0.1%	Initiated Phase 1/2a study of dual-target RNAi candidate ARO-DIMER-PA.
2026-01-20	Earnings call scheduling	Neutral	+3.5%	Announced webcast and call date for fiscal 2026 Q1 results discussion.
2026-01-07	Convertible and equity deal	Negative	-1.1%	Priced sizable 0.00% convertible notes and common stock/pre-funded warrant offerings.

24h Move is the share-price change in the day after each event; other market factors may also have contributed.

Pattern Detected

Recent ARWR news has mostly seen modest price reactions, with capital-raising news aligning negatively and strong earnings producing a slight divergence.

Recent Company History

Over recent months, Arrowhead has combined financing, clinical progress, and a major earnings turnaround. January 2026 saw upsized convertible and equity offerings, followed by initiation of a Phase 1/2a cardiometabolic study. A strong fiscal 2026 Q1, with significantly higher revenue and a net profit, produced only a small negative move. Subsequent conference and investor event updates drew modest gains. Today’s S&P index reclassification adds a new structural element to that trajectory.

Key Terms

s&p 500, s&p midcap 400, s&p smallcap 600, dow jones industrial average, +1 more

5 terms

s&p 500 financial

"Ciena Corp. (NYSE: CIEN) will replace Dayforce Inc. (NYSE: DAY) in the S&P 500"

The S&P 500 is a broad stock market index that tracks the performance of 500 large U.S. companies, weighted so bigger firms have a larger impact. Investors use it like a thermometer or benchmark to judge how the overall U.S. stock market or a portfolio is doing; movements in the index influence investor sentiment, fund performance, and many passive investment products that aim to match its returns.

View in glossary

s&p midcap 400 financial

"Arrowhead Pharmaceuticals Inc. (NASD: ARWR) will replace Ciena in the S&P MidCap 400"

The S&P MidCap 400 is a group of 400 companies that are smaller than the big, well-known corporations but larger than tiny startups. It helps investors see how medium-sized companies are performing in the stock market, acting like a barometer for this middle-sized group of businesses.

s&p smallcap 600 financial

"ADT Inc. (NYSE: ADT) will replace Arrowhead Pharmaceuticals in the S&P SmallCap 600"

A stock market index made up of 600 small-cap U.S. companies chosen to represent the performance of smaller publicly traded firms. It matters to investors because it acts like a temperature gauge for the small-company segment of the market, serving as a benchmark for fund managers, a basis for index funds and ETFs, and a way to compare returns and risk when building a diversified portfolio.

dow jones industrial average financial

"such as the S&P 500® and the Dow Jones Industrial Average®."

A stock market index that tracks the share prices of a small group of large, well-known companies to give a quick snapshot of how the broader market is doing. Think of it like a thermometer for the stock market: it uses a handful of heavyweight firms as representative samples to show whether investor sentiment is rising or falling, which helps investors gauge market trends and compare performance over time.

View in glossary

gics sector financial

"Effective Date | Index Name | Action | Company Name | Ticker | GICS Sector"

GICS sector is one of the major industry groupings from the Global Industry Classification Standard, a system that sorts companies into consistent categories (like Technology or Health Care) based on the primary business they do. Investors use these sectors like folders in a filing cabinet to compare companies, build diversified portfolios, and track how whole parts of the market are performing rather than focusing on single stocks.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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NEW YORK, Feb. 4, 2026 /PRNewswire/ -- S&P Dow Jones Indices will make the following changes to the S&P 500, S&P MidCap 400, S&P SmallCap 600: 

• S&P MidCap 400 constituent Ciena Corp. (NYSE: CIEN) will replace Dayforce Inc. (NYSE: DAY) in the S&P 500, S&P SmallCap 600 constituent Arrowhead Pharmaceuticals Inc. (NASD: ARWR) will replace Ciena in the S&P MidCap 400, and ADT Inc. (NYSE: ADT) will replace Arrowhead Pharmaceuticals in the S&P SmallCap 600 effective prior to the opening of trading on Monday, February 9. Thoma Bravo L.P. acquired Dayforce in a deal that closed today.
• OneSpaWorld Holdings Ltd. (NASD: OSW) will replace Dynavax Technologies Corp. (NASD: DVAX) in the S&P SmallCap 600 effective prior to the opening of trading on Tuesday, February 10. Sanofi is acquiring Dynavax Technologies in a deal expected to be completed soon pending final closing conditions.

Following is a summary of the changes that will take place prior to the open of trading on the effective date:

Effective Date	Index Name	Action	Company Name	Ticker	GICS Sector
Feb 9, 2026	S&P 500	Addition	Ciena	CIEN	Information Technology
Feb 9, 2026	S&P 500	Deletion	Dayforce	DAY	Industrials
Feb 9, 2026	S&P MidCap 400	Addition	Arrowhead Pharmaceuticals	ARWR	Health Care
Feb 9, 2026	S&P MidCap 400	Deletion	Ciena	CIEN	Information Technology
Feb 9, 2026	S&P SmallCap 600	Addition	ADT	ADT	Consumer Discretionary
Feb 9, 2026	S&P SmallCap 600	Deletion	Arrowhead Pharmaceuticals	ARWR	Health Care
Feb 10, 2026	S&P SmallCap 600	Addition	OneSpaWorld Holdings	OSW	Consumer Discretionary
Feb 10, 2026	S&P SmallCap 600	Deletion	Dynavax Technologies	DVAX	Health Care

ABOUT S&P DOW JONES INDICES

S&P Dow Jones Indices is the largest global resource for essential index-based concepts, data and research, and home to iconic financial market indicators, such as the S&P 500® and the Dow Jones Industrial Average®. More assets are invested in products based on our indices than products based on indices from any other provider in the world. Since Charles Dow invented the first index in 1884, S&P DJI has been innovating and developing indices across the spectrum of asset classes helping to define the way investors measure and trade the markets.

S&P Dow Jones Indices is a division of S&P Global (NYSE: SPGI), which provides essential intelligence for individuals, companies, and governments to make decisions with confidence. For more information, visit www.spglobal.com/spdji/en/. 

FOR MORE INFORMATION:

S&P Dow Jones Indices
index_services@spglobal.com

Media Inquiries
spdji.comms@spglobal.com

View original content:https://www.prnewswire.com/news-releases/ciena-set-to-join-sp-500-arrowhead-pharmaceuticals-to-join-sp-midcap-400-adt-and-onespaworld-holdings-to-join-sp-smallcap-600-302679569.html

SOURCE S&P Dow Jones Indices

FAQ

When will ADT (ADT) join the S&P SmallCap 600?

ADT will join the S&P SmallCap 600 prior to the opening of trading on Feb 9, 2026. According to S&P Dow Jones Indices, this is part of a scheduled rebalancing affecting multiple indexes and takes effect at the market open on that date.

Why was Dayforce (DAY) removed from the S&P 500 on Feb 9, 2026?

Dayforce was removed from the S&P 500 because Thoma Bravo completed an acquisition of Dayforce. According to S&P Dow Jones Indices, the deal closed and the company no longer met index inclusion criteria, prompting the deletion effective Feb 9, 2026.

What does Ciena's (CIEN) addition to the S&P 500 mean for investors?

Ciena's addition places CIEN in the S&P 500 effective Feb 9, 2026, which may increase passive fund demand. According to S&P Dow Jones Indices, index inclusion typically prompts reweighting by index-tracking funds at the open on the effective date.

How does Arrowhead Pharmaceuticals (ARWR) moving to the S&P MidCap 400 affect its index status?

Arrowhead will be added to the S&P MidCap 400 and removed from the S&P SmallCap 600 prior to trading on Feb 9, 2026. According to S&P Dow Jones Indices, the change reflects index reconstitution and will alter which funds track ARWR.

When does OneSpaWorld (OSW) join the S&P SmallCap 600 and what is the effective date?

OneSpaWorld will join the S&P SmallCap 600 effective prior to the opening of trading on Feb 10, 2026. According to S&P Dow Jones Indices, OSW replaces Dynavax in the SmallCap 600 on that effective date.

Was the Dynavax (DVAX) removal tied to a transaction, and what is the timing?

Dynavax was deleted from the S&P SmallCap 600 effective Feb 10, 2026 ahead of a pending acquisition by Sanofi. According to S&P Dow Jones Indices, the removal aligns with the announced transaction awaiting final closing conditions.