Guide

Secondary offering explained

Harbor Biotech announced a $340 million follow-on equity offering in March 2026: $220 million of primary shares (new stock sold by the company for R&D and manufacturing scale-up) and $120 million of secondary shares (existing holders — two early VC funds and the CEO — selling into the deal). The stock closed at $48.20 the day before pricing; the underwriters set the offer at $45.20, a 6.2% discount. Retail holders focused on the 8.1% dilution from primary shares alone. What they missed: the 4.1 million secondary shares still expanded the public float by 11%, pushed Harbor into the mid-cap index band, and triggered passive fund rebalancing that added two days of selling pressure after the deal. The CFO’s board deck had modeled only primary dilution — a $0.14 EPS guidance miss when fully diluted share count and higher G&A from the offering roadshow were included.

Colloquially, a secondary offering means any sale of stock after an IPO. In underwriting language, “secondary” often means shares sold by existing holders, while “primary” means new shares issued by the company. Most follow-on deals mix both. This guide clarifies that vocabulary, walks through shelf registrations, overnight marketed offerings, block trades, and at-the-market (ATM) programs, shows dilution and float math, covers underwriting fees and the greenshoe, presents the Harbor Biotech walkthrough, compares financing vehicles in a decision table, lists pitfalls, and ends with an investor and issuer checklist.

Primary vs secondary shares

Primary shares are newly created stock the company sells to investors. Cash goes to the company’s balance sheet. Share count rises, so existing holders are diluted unless the company deploys capital into value-creating projects that lift per-share metrics.

Secondary shares are sold by existing shareholders — founders, employees exercising options, or funds distributing portfolio positions. Cash goes to those sellers, not the company. The company’s share count does not increase from secondaries alone, but the tradable float does, which changes liquidity, index eligibility, and supply/demand dynamics.

A mixed offering packages both: common in biotech after Phase 2 data, when the company needs primary capital and early investors want partial liquidity without a separate block sale. Headlines often say “$500M offering” without splitting the mix — read the prospectus supplement.

Do not confuse this with a stock buyback, which retires shares, or a stock split, which rescales count without changing economics.

Common follow-on structures

Shelf registration (Form S-3)

Mature issuers with public float and reporting history file a shelf registration on Form S-3, registering a pool of securities (e.g. $1 billion of common stock) for up to three years. When they want to raise, they file a short prospectus supplement with deal size, use of proceeds, and underwriting spread. Shelves let companies move quickly when windows open — after earnings beats, favorable FDA headlines, or sector rallies.

Overnight / marketed follow-on

The company and lead underwriters announce after the close, launch a roadshow or institutional soundings overnight, and price before the next open. Discounts to the unaffected price typically run 3–8% depending on size, volatility, and demand. Large deals in thin floats widen discounts.

Block trade

A holder sells a large block to the underwriter at a fixed discount; the underwriter resells to institutions, often without a company primary component. Block trades can pressure the stock even when no primary dilution occurs.

At-the-market (ATM) program

An ATM lets the company dribble primary shares into the market at prevailing prices through a sales agent, often off the same shelf. ATMs minimize headline shock and discount but can create a persistent overhang; disclosure appears in quarterly filings as shares sold under the program.

Registered direct / PIPE-style public deals

A small set of institutional investors buys a registered primary block, sometimes with warrants. Priced at a modest discount with lighter underwriting fees than a full follow-on. Distinct from private PIPEs in pre-public companies.

Dilution and float math

Primary dilution percentage ≈ new primary shares ÷ (pre-deal shares outstanding + new primary shares). Harbor had 248 million shares outstanding pre-deal; $220M primary at $45.20 issued ~4.87 million new shares:

4.87 ÷ (248 + 4.87) ≈ 1.9% accounting dilution from primary alone. If you measure against fully diluted shares including options and RSUs (272 million), primary dilution is ~1.7% — but option dilution was already in the cap table; the marginal hit to existing holders is still the new primary slice.

Secondary shares in Harbor’s deal: 4.1 million. No change to shares outstanding, but float rose from ~180 million to ~184 million (+2.3%). Combined with primary, tradable supply jumped ~5%. That matters for market cap index rules, short interest as a percent of float, and volume-weighted impact.

Use of proceeds tells you whether dilution buys growth: Harbor allocated 65% to manufacturing capex, 25% to Phase 3 trials, 10% to working capital — reasonable for a biotech scaling post-data. Dilution without credible ROI destroys per-share value.

Underwriting, fees, and the greenshoe

Lead underwriters earn a gross spread (often 3–5% for follow-ons, tighter for large liquid issuers). The spread compensates banks for taking distribution risk if institutional demand softens. Secondary portions still pay underwriting fees; sellers net the offer price minus spread.

The greenshoe (overallotment option) lets underwriters sell up to 15% extra shares (usually primary) if demand exceeds the base deal, stabilizing post-offering trading. If the stock trades up, banks exercise the shoe and buy stock in the open market to cover shorts. If it trades down, they may support the price with stabilization bids (within regulatory limits).

Lock-up releases from prior IPOs or acquisitions can coincide with follow-ons. Even when secondaries are small, upcoming lock-up expiries act as supply overhang and widen required discounts.

Regulatory and disclosure essentials

Follow-ons rely on current SEC reporting: the base shelf (S-3) and prospectus supplement describe risk factors, material changes, and intended use of proceeds. Material nonpublic information triggers timing constraints — companies cannot price while sitting on undisclosed trial results or merger talks without updating disclosure.

Investors should read: (1) supplement use-of-proceeds split primary vs secondary; (2) whether insiders participate as sellers; (3) change in share authorization if the company is near its charter limit; (4) updated share count and net cash pro forma; (5) any warrant or convertible overhang from prior deals.

Harbor Biotech follow-on walkthrough

Pre-offering: 248M basic shares, $48.20 close, ~$11.9B market cap. Deal: 10.8M total shares at $45.20 — 4.87M primary ($220M to company), 4.10M secondary ($185M to selling holders), 0.83M greenshoe primary option.

Pro forma cash: $180M on balance sheet pre-deal + $220M primary − ~$12M fees ≈ $388M. Runway extended from 14 to 28 months at current burn. Secondary sellers: Fund A ($90M), Fund B ($70M), CEO ($25M) — all disclosed as liquidity diversification, not a clinical setback.

Stock reaction: −9.4% day one (discount + supply), partial recovery day three. Passive mid-cap index adds completed by day five. EPS model error: board used 252.9M shares (primary only) instead of 252.9M basic + ongoing RSU vesting + potential shoe — understated Q3 share count by 3.2M, a $0.14 EPS miss versus guidance.

Lesson: model fully diluted count including ATMs, RSU grants, and greenshoe probability; treat secondary float expansion as a trading input even when accounting dilution is zero.

Technique decision table

Vehicle	Cash to company	Dilution / supply	Best when
Primary follow-on (overnight)	Large lump sum	Primary dilution + headline discount	Fund M&A, capex, trials; window after positive catalyst
Secondary-only block	None	Float expansion, no share-count increase	Insider/fund liquidity; company does not need cash
ATM program	Gradual	Slow drip dilution, persistent overhang	Opportunistic raise in volatile stock; avoid big discount
Convertible bond / note	Debt + optional equity	Future conversion dilution; interest expense	Tax shield, patient capital; stock at premium to convert price
Stock buyback	Cash outflow	Reduces shares; supports EPS	Undervalued stock, excess FCF, no high-ROI internal projects
Bank debt / revolver	Debt funding	No immediate equity dilution	Profitable or asset-backed; covenants acceptable

Common pitfalls

Ignoring secondary float — zero accounting dilution does not mean zero price impact when 10% of float hits the market.
Primary-only models — Harbor’s EPS miss from incomplete fully diluted share count is typical of rushed board decks.
Chasing windows too late — pricing after a 40% rally into a shelf often requires a steep discount that wipes the momentum benefit.
ATM opacity — investors discover months of dribble sales only in 10-Q footnotes; disclose pace and remaining capacity clearly.
Insider secondary optics — CEO selling in a mixed deal without a 10b5-1 plan invites governance questions even when size is modest.
Authorization ceiling — charter share limits force a shareholder vote mid-deal, delaying closings and widening discounts.
Confusing IPO secondaries — at IPO, “secondary” means selling shareholders cash out; at follow-ons, primary usually dominates use-of-proceeds.
Index churn neglect — float and market-cap band changes move passive flows independent of fundamentals for 1–2 weeks post-deal.

Production checklist

Split primary vs secondary in headline IR and prospectus summary.
Model pro forma cash, burn runway, and fully diluted share count including shoe.
Disclose seller identities, shares sold, and remaining holdings for insiders.
File shelf well before you need it; keep S-3 eligibility current.
Benchmark discount to peer follow-ons of similar size and volatility.
Coordinate with index providers on float and free-float changes if material.
Update EPS guidance for higher share count and one-time offering costs.
Monitor ATM sales monthly in investor updates, not only in 10-Q.
Plan lock-up and RSU vest calendars to avoid stacking supply events.
Post-deal: track volume, short interest, and passive flow for two weeks.

Key takeaways

Primary shares dilute ownership; secondary shares expand float without raising company cash — both affect the stock.
Shelf S-3 registrations let issuers move quickly; ATMs trade headline risk for gradual dilution.
Discounts compensate underwriters and buyers for supply shock; greenshoes stabilize trading afterward.
Harbor Biotech showed how modeling primary dilution alone misses float, index, and EPS impacts.
Choose follow-ons when ROI on new capital exceeds the cost of dilution; choose buybacks when the opposite holds.